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Strix occidentalis

• INTRODUCTORY
• WILDLIFE DISTRIBUTION AND OCCURRENCE
• BIOLOGICAL DATA AND HABITAT REQUIREMENTS
• FIRE EFFECTS AND USE
• REFERENCES

INTRODUCTORY

AUTHORSHIP AND CITATION FEIS ABBREVIATION COMMON NAMES TAXONOMY SYNONYMS ORDER CLASS FEDERAL LEGAL STATUS OTHER STATUS
	Northern spotted owl. United States Fish and Wildlife Service image.

AUTHORSHIP AND CITATION:
Meyer, Rachelle. 2007. Strix occidentalis. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: www.fs.usda.gov/database/feis/animals/bird/stoc/all.html [].

FEIS ABBREVIATION:
STOC

COMMON NAMES:
spotted owl

TAXONOMY:
Strix occidentalis (Xántus) is the scientific name of the spotted owl, a member of the Strigidae family [8]. The three recognized subspecies of spotted owl are:

Strix occidentalis caurina (Merriam) northern spotted owl
Strix occidentalis occidentalis (Xántus) California spotted owl
Strix occidentalis lucida (Nelson) Mexican spotted owl [7].

Throughout this review subspecies are referred to by the common names used above.

Although apparently rare, northern spotted owl × northern barred owl (Strix varia varia) hybrids have been detected [86,150,161].

SYNONYMS:
None

ORDER:
Strigiformes

CLASS:
Bird

FEDERAL LEGAL STATUS:
The northern and Mexican spotted owl are listed as Threatened. The California spotted owl is Under Review for listing [194].

OTHER STATUS:
Information on state-level protected status of animals in the United States is available at NatureServe, although recent changes in status may not be included.

ANIMAL DISTRIBUTION AND OCCURRENCE

• GENERAL DISTRIBUTION
• STATES/PROVINCES
• PLANT COMMUNITIES

GENERAL DISTRIBUTION:
The following distribution information was obtained from several reviews [7,83,84,134,192,197]. Spotted owls occur from British Columbia south to California and Texas, and east to Colorado. They do not occur in the Northern Rocky Mountains. The nearly contiguous range of the northern spotted owl extends from southwestern British Columbia south through western Washington and Oregon to Marin County on the north-central coast of California [7,83,84,134]. The California spotted owl's range overlaps that of the northern spotted owl in the southern Cascade Range, and extends south through the western Sierra Nevada to Tulare County. They also occur in discrete populations in coastal mountainous areas Monterey County, California, to northern Baja California [7,83,84,134,197]. In the United States the Mexican spotted owl occurs in disjunct populations in mountain ranges and canyons of Utah, Colorado, Arizona, New Mexico, and extreme western Texas. In Mexico it ranges from Sonora, Chihuahua, Nuevo León, and eastern Coahuila through the Sierra Madre Occidental and Sierra Madre Oriental as far south as Michoacán [7,83,84,134,192,201]. NatureServe provides a distributional map for the spotted owl.

Overall distribution of spotted owl. Map courtesy of Nk, Creative Commons.

California spotted owls occur in hardwood, coniferous, and coniferous-hardwood forests. Occupied coniferous habitats include mixed conifer [30,183,189,197,199,203], California red fir (A. magnifica) [189,199,203], and eastside pine forests which are comprised of ponderosa pine and/or Jeffrey pine (Pinus jeffreyi) [189,199]. Redwood/California bay (Umbellularia californica) [183,199], ponderosa pine/hardwood [189,199], and live oak-bigcone Douglas-fir (Quercus chrysolepis or Q. agrifolia-Psuedotsuga macrocarpa) [183,197,199,203] are hardwood-mixed coniferous forests used by California spotted owls. They also occur in hardwood habitats including riparian [83,183,189,197,199,203] and oak (Quercus spp.) woodlands [83,157,184]. For example, in the Tehachapi Mountains of southern California they occurred in stands dominated by canyon live oak (Q. chrysolepis) [26]. California spotted owls have also been observed in chaparral communities [208].

Mexican spotted owls occur in varied habitats. Ponderosa pine-Gambel oak (Q. gambelii) [20,76,79,205] and mixed-conifer forests, typically dominated by Douglas-fir and/or white fir (Abies concolor) [20,30,78,83,205,206], are often used. In Arizona, ponderosa pine-Gambel oak vegetation was selected [74], and roosting Mexican spotted owls in New Mexico generally preferred mixed-conifer and mixed-conifer/oak forests [93]. The majority of roosting sites in southern Arizona were in mixed-conifer or pine-oak habitats, but some occurred in Madrean evergreen woodland and interior chaparral [205]. Pinyon-juniper (Pinus spp.-Juniperus spp.) woodlands provide nonbreeding habitat and may be used to some extent during the breeding season [73,78,79,83,206]. However, in New Mexico, pinyon-juniper and open ponderosa pine woodlands were avoided [93]. Other woodlands used by Mexican spotted owls include riparian woodland [20,78,206], encinal oak (Q. emoryi, Q. arizonica, Q. oblongifolia, Q. grisea [56,112]) woodland [79,192], pine (Pinus leiophylla, P. engelmannii, P. ponderosa, P. strobiformis) woodland with evergreen oak (e.g. Q. chrysolepis) understories [79,83], and Arizona cypress (Cupressus arizonica) woodland [79]. Montane meadows are used to some extent for foraging [72,200,216]. Habitats such as mountain shrub and desert scrub are used during the winter by dispersing juveniles and possibly migratory adults [73,206].

BIOLOGICAL DATA AND HABITAT REQUIREMENTS

• TIMING OF MAJOR LIFE HISTORY EVENTS
• PREFERRED HABITAT
• COVER REQUIREMENTS
• FOOD HABITS
• PREDATORS
• MANAGEMENT CONSIDERATIONS

Breeding Cycle: The majority of information on the spotted owl's breeding cycle is from a study of northern spotted owls in western Oregon [62]. General timing of breeding events is likely similar for California [197] and Mexican spotted owls [192]. A review of the California spotted owl's biology and ecology notes that those in southern California typically breed slightly earlier than those in the Sierra Nevada [197].

The spotted owl breeds in early spring, nests in spring, and fledgesin summer, and disperses in fall. Breeding spotted owls begin prelaying behaviors, such as preening and roosting together, in February or March. In western Oregon, laying occurred from 9 March to 19 April, with an average date of 2 April [62]. Northern spotted owl eggs in western Oregon were incubated for about 30 days and hatched from 8 April to 20 May. Hatchlings fledged after 34 to 36 days and reached independence in August and September [62]. In coniferous forests of north-central Arizona, average time from fledgling to independence for Mexican spotted owls was 87 days in one year and 101 days in another [75]. In western Oregon, dispersal of juvenile (young of the year) northern spotted owls was documented in October [62] and from early September to early November [139]. Information on California spotted owl dispersal is sparse. A review summarizes studies of northern spotted owl dispersal and suggests that dispersal of both subspecies is likely to occur in September and October [197]. Mexican spotted owls usually disperse in September, but have been observed dispersing from August to October [75,93,205].

For more information on breeding behavior such as pair bonding, parental care, and territoriality see [62] or the review [83], which also summarizes spotted owl information related to temperature regulation and vocalizations.

Brief literature reviews of the spotted owl's life history also note that the spotted owl clutch size is typically 2 eggs, but can be 1, 3, or very rarely 4 eggs [82,134]. The average clutch size of northern spotted owls in western Oregon was 2 eggs [62].

Spotted owl reproductive output is variable. In Douglas-fir/hardwoods, mixed-conifer, and Oregon white oak (Quercus garryana) forests of northwestern California, average annual northern spotted owl reproductive output ranged from 0.150 to 0.810 fledged young/pair [64]. The percentage of years (n=5) in which pairs in coniferous forests and Douglas-fir/hardwood woodlands produced at least 1 fledging ranged from 0 to 80% [188]. In a study of both northern and California spotted owls in California, fledging rate varied from 0 to 100% over 8 years and 2 study sites [16]. In oak woodland and coniferous forests of the southern Sierra Nevada, the average annual reproductive output of California spotted owl varied from 0.07 to 1.67 young/pair over a 9-year period [157].

Several factors contribute to spotted owl's variable reproductive success. For instance, spotted owl pairs do not breed every year [62,192,197]. Over a 5-year period in western Oregon, the percentage of nesting northern spotted owls averaged 62% and ranged from 16% to 89% [62]. A review states that the percentage of California spotted owl pairs that attempt to breed in a given year ranges from essentially none to almost all pairs nesting [197].

Another source of the variation in reproductive output is spotted owl age. Spotted owls are typically reproductive by 3 years of age. They may, albeit rarely, breed as young as 1 year old [60,62]. In Washington and Oregon, the majority of radio-marked northern spotted owls were paired by 2 years of age, and recruitment of banded owls into the territorial population occurred at an average age of 2.4 years [60]. The average number of fledglings produced by 1-year-old northern spotted owls in 11 study areas across their range was 0.074 fledglings/territorial female. Two-year-olds fledged an average of 0.208 young/territorial female, and individuals 3 years and older fledged an average of 0.372 young/territorial female [127]. Increased reproductive output of northern spotted owls 3 years or older compared to juveniles and subadults (1 and 2 year olds) has been observed in several study areas [13,64]. In Arizona and New Mexico, Mexican spotted owls 2 years and older had greater fecundity than 1-year-olds [173].

Site characteristics can also affect the reproductive output of spotted owls [19,77,104]. Increased elevation was negatively associated with reproductive success of northern spotted owls in coniferous forests of the eastern Cascade Range in Washington [104], and of California spotted owls in coniferous forests in northeastern California [24]. See the Elevation section for more details. In areas throughout their range with less than 20% suitable habitat, northern spotted owls averaged only 0.33 fledglings/pair, while in areas with more than 60% suitable habitat they averaged 0.93 fledglings/pair [19]. Northern spotted owls in nonglaciated montane slopes of the eastern Cascade Range of Washington averaged 0.57 fledglings/year, significantly (P<0.01) more than those in glacially scoured regions, which averaged 0.38 fledgling/year [104]. In the Sacramento Mountains of New Mexico, Mexican spotted owls in mixed-conifer forests had higher average annual reproductive output (0.38 female fledged/territory) than those in ponderosa pine and twoneedle pinyon-alligator juniper (Pinus edulis-Juniperus deppeana) woodlands (0.13 female fledged/territory) [77].

Weather has a strong influence on spotted owl reproductive success. Good weather, such as lower precipitation and higher minimum temperatures during the nesting season, is associated with higher reproduction [64,99,157,185]. For more detail on the effect of weather on reproduction, survival, habitat selection, and site occupancy see Weather.

Food may also play a role in spotted owl reproductive success, with larger average prey associated with higher breeding rates. For more information on the effect of diet on breeding status and nesting success see Food Habits.

Dispersal: Juvenile spotted owls disperse in late summer to fall (see Breeding Cycle). Dispersing juvenile northern spotted owls in Oregon and Washington typically settled in a wintering range from October or November until February to April, and from there dispersed to a breeding-season home range. Some individuals remained in this home range, while others occupied a series of temporary home ranges until settling sometime from 2 to 5 years of age [60]. For a discussion of the habitats spotted owls travel through while dispersing see Dispersal Habitat.

Although variable, most spotted owls disperse less than 19 miles (30 km) [60,75,206]. See the table below for information on dispersal distances of northern and Mexican spotted owls. Only 8.7% of northern spotted owls in Washington and Oregon dispersed over 31 miles (50 km). According to a review on the California spotted owl, the limited information on distances suggests a pattern similar to the other subspecies [197].

Occasionally adult (>2 years old) spotted owls disperse from their territories. The most thorough investigation of this behavior found an annual average adult northern spotted owl dispersal rate of 6.6% in Oregon and Washington. Females, 3- to 4-year-old adults, and individuals with no mate in the current or previous season were most likely to disperse from their territory. The average adult dispersal distance was 3.8 miles (6.1 km) [60]. In the San Bernardino Mountains, no more than 20% of California spotted owls 1 year old or older emigrated [215]. The recovery plan notes the high fidelity of Mexican spotted owls to their territories and includes persistence rates as high as 83% in the Basin and Range- West Recovery Unit. This represents the proportion of individuals that remain on their territory from one year to the next. Whether a territory was abandoned or the owl within it died was not distinguished [192].

For information on the metapopulation structure of California spotted owls in southern California see [153] and for a review of Mexican spotted owl metapopulation information see [110].

Life Span: Spotted owls are long lived. According to a literature review of spotted owl life history and ecology in a bird handbook, 16- to 17-year-old northern spotted owls have been documented in Oregon [134]. Average annual survival rate estimates are usually more than 0.8 for adults of all 3 subspecies [29,64,65,173], although average annual survival rates as low as 0.75 have been reported for northern spotted owls [13,127].

Survival rates of juvenile and subadult spotted owls are low and generally more variable than adult survival rates [13,29,127,173]. A meta-analysis of northern spotted owl data estimated annual juvenile survival rates at 21.2% to 29.2%. The authors caution that these values were likely underestimates [29]. Annual survival rates of 1-year-old northern spotted owls from 11 study areas throughout their range varied from 42% to 86%, while annual survival rates of 2-year-olds ranged from 63% to 89% [127]. In New Mexico, Mexican spotted owls from 1 to 2 years old had average annual survival rates of 64.4%. Mexican spotted owls less than a year old had average annual survival rates of 11% in New Mexico and 17.9% in Arizona [173].

Survival rates may vary with habitat. For instance, analysis of northern spotted owl data found increased (P≤0.03) persistence, an index of survival, in areas with greater amounts of older (>120 years) forests [18]. In the Sacramento Mountains of New Mexico, Mexican spotted owls in mixed-conifer forest had higher survival than in ponderosa pine or pinyon-juniper forests, but the difference was not statistically significant (P=0.194, n=6) [77].

The most common causes of spotted owl mortality are predation and starvation [60,77]. Most juvenile northern spotted owls in Washington and Oregon died from predation (68%) or starvation (26.2%) [60]. Both adult and juvenile Mexican spotted owls in New Mexico died from starvation and predation [77]. According to a comprehensive review, juveniles also die from exposure [83]. Accidents are an additional source of mortality. Given that 67% of northern spotted owls in Washington and Oregon that died from other causes were diseased, illness may increase the risk of mortality [60].

Elevation: Spotted owls occur at a range of elevations, with higher elevations occupied at lower latitudes.

Northern spotted owls occur at elevations from 70 to 6,600 feet (20-2,010 m), with the majority in the lower portions of this range [32,62,84]. In coniferous forests of northwestern California, nest sites ranged from 118 to 4,944 feet (35-1,507 m), with 94% occurring below 4,000 feet (1,218 m) [114]. In mixed evergreen and mixed-conifer forests of northwestern California, roosting northern spotted owls avoided areas above 2,950 feet (900 m) [23]. In coniferous forests of the Klamath, Coast and Cascade regions in Oregon and the Olympic peninsula of Washington, nest locations were significantly lower (P<0.001) in elevation than random sites within northern spotted owl's home ranges [91]. In coniferous forests of southwestern Washington, important owl locations (e.g., nest sites, multiple detection sites) averaged 3,170 feet (966.2 m), which was significantly (P<0.001) lower than the 3,510-foot (1,070.3 m) average elevation at random sites [161]. In coniferous forests of the eastern Cascade Range of Washington, elevation of northern spotted owl nest sites was negatively associated with latitude (P<0.001) [32], and site occupancy and reproductive rates were inversely associated with elevation [104].

According to reviews, California spotted owls occur on sites from about 1,000 to 8,500 feet (300-2,600 m), with individuals in southern California generally occurring at higher elevations [84,197]. A detailed summary of California spotted owl habitat associations reports nest sites occurring at an average elevation of 5,300 feet (1,620 m) in the northern Sierra Nevada and 6,000 feet (1,830 m) in southern California [85]. In white fir-mixed-conifer stands of the Lassen National Forest in northeastern California, elevation at California spotted owl nest areas was inversely associated with site occupancy and reproductive output [24].

According to a review and analysis of habitat relationships in the Mexican spotted owl recovery plan, 95% of Mexican spotted owl nests in Arizona and New Mexico study sites were in trees at elevations from 6,000 to 8,500 feet (1,829-2,591 m), with 72% occurring from 6,500 to 7,500 feet (1,982 and 2,287 m) [78]. In Colorado, Mexican spotted owls occurred from 5,820 to 9,100 feet (1,770-2,770 m), with 17 of 20 from 6,500 to 7,800 feet (1,980-2,380 m) [49]. In Saguaro National Park, Mexican spotted owl territories did not occur below 7,000 feet (2,100 m) [205]. In another southern Arizona study site, Mexican spotted owl nest/roost sites occurred from 5,820 to 7,620 feet (1,773-2,323 m) [46].

Topography: Effects of aspect on northern spotted owl habitat selection are ambiguous. Aspects at nesting and roost sites in mixed evergreen and mixed-conifer forests of northwestern California did not differ from availability [23]. In the western Cascade Range of southwestern Washington, aspects of spotted owl sites did not differ significantly (P>0.05) from random sites [161]. Similarly, data from low to mid-elevation forests of northwestern California also indicated that aspects at nest sites did not differ from random sites [114]. In southwestern Oregon, north to east aspects were used more frequently and south to southwest aspects were used less frequently than expected in summer. In contrast, spring and fall roost sites occurred more frequently on south and southwest aspects (P<0.05) [42]. The aspect at nest sites in the eastern Cascade Range of Washington averaged 35? northeast, which was significantly (P=0.015) different from the average aspect of 48? northeast on random sites in the nest stand [32]. Over 50% of summer roosting and foraging observations occurred on north-facing slopes in predominantly mixed-evergreen forests of northwestern California [180].

California and Mexican spotted owls may select northern slopes and/or avoid southern slopes. In the central Sierra Nevada, roost sites, but not nest sites, faced north (x=16? north) significantly (P<0.05) more often than a uniform distribution [22]. One of the many factors associated with higher reproductive rates in oak woodland of the southern Sierra Nevada was nesting on north-facing slopes [157]. Although likely confounded with vegetation characteristics, California spotted owl nests did not occur on sites with southern aspects in foothill riparian and oak woodlands in the southern Sierra Nevada [184]. However, the average aspects of nesting and roosting sites in the San Bernardino Mountains were not significantly different than average aspects of random sites, according to a detailed summary of California spotted owl habitat studies [85]. In southern Arizona, 7 out of 10 of Mexican spotted owl nest/roost sites were on northwest facing slopes [46]. In Saguaro National Park most roost sites occurred on northwest facing slopes [205]. According to a review and analysis of habitat relationships in the Mexican spotted owl recovery plan, nearly 50% of Mexican spotted owl nests occurred on north or northeast aspects in Arizona and New Mexico study sites [78].

Spotted owls seem to select the lower portions of slopes, at least in summer. Although most of the data are from studies on the northern spotted owl [23,42,59,62,91,114], there is evidence that California [184] and Mexican spotted owls [78,93] also select slope bases. In mixed-evergreen and mixed-conifer forests of northwestern California, northern spotted owls nested and roosted on the lower third of slopes significantly more, the middle third in proportion with, and the upper third of slopes significantly less than expected based on availability during the breeding season (α=0.05) [23]. On sites in coniferous forests in Oregon and Washington, 95% of nest sites were on the bottom or middle third of slopes, although this was only significantly (P<0.025) more than random sites within home ranges in the Klamath Mountains [91]. In southwestern Oregon, lower slopes were used significantly more than expected for roosting during summer. However, in the spring, fall, and winter northern spotted owl used upper and mid-slopes significantly more than expected (P<0.01) [42]. In foothill riparian and oak woodlands in the southern Sierra Nevada, California spotted owl nest sites were typically on the lower third of slopes [184]. Foraging Mexican spotted owls in mixed-conifer, mixed-conifer/oak, ponderosa pine, and pinyon-juniper habitats of southwestern New Mexico selected canyon bottoms or the lower portion of slopes significantly (P<0.05) more than expected in 3 study areas [93]. According to a review of habitat relationships in the Mexican spotted owl recovery plan, nearly half of Mexican spotted owl nests on Arizona and New Mexico study sites occurred on the lower third of slopes [78].

In coniferous forests of the eastern Cascade Range of Washington, topography may influence northern spotted owl reproductive success. In glacially-scoured, topographically-varied landscapes, average reproductive output was 0.38 fledglings/year, while in more gently rolling, montane slopes it averaged 0.57 fledglings/year (P<0.01) [104].

Water: In some regions, northern spotted owls use areas near water. In mixed-evergreen forests of northwestern California, the summer roost sites of 10 northern spotted owls averaged 466 feet (142.1 m) from water, which was significantly (P<0.01) shorter than the average 743 feet (226.6 m) from random locations to water [180]. In managed timberlands in the coastal redwood vegetation zone of northwestern California, northern spotted owl nest areas were closer to water than randomly-selected plots (P=0.032) [59]. Nest sites in low- to mid-elevation conifer forests of northwestern California averaged 385 feet (117.3 m) from water [114]. On 2 sites in the Coast and Cascade Ranges in western Oregon, 84% of nests were within 820 feet (250 m) of a stream or spring [62]. In southwestern Oregon, roost sites were significantly (P<0.01) closer to water in summer (x = 240 feet (74 m)) than in winter (x = 325 feet (99 m)) [42]. A literature review states that Mexican spotted owls occur in canyons with perennial water sources [83].

Weather: Reproductive rates of northern [64,104] and California [157] spotted owls are strongly influenced by weather. Low northern spotted owl reproductive output in Douglas-fir/hardwood and mixed-conifer forests of northwestern California was associated with cold, wet springs [64]. Average productivity in the eastern Cascade Range of Washington also declined with increasing precipitation, from 0.10 young/year on sites receiving more than 118 inches (300 cm) of precipitation a year to 0.96 young/year on sites receiving less than 20 inches (51 cm) of precipitation a year [104]. High California spotted owl reproductive rates were associated with less precipitation and higher minimum temperatures during the breeding season (March-May). For example, in mixed-conifer forests, reproductive output averaged 1.585 fledglings/pair in breeding seasons with less than 8 inches (20.7 cm) of rain and 0.307 fledglings/pair in breeding seasons with more than 8 inches of rain. In breeding seasons with more than 8 inches of rain, California spotted owl reproductive output was greater when the minimum April temperature was above 28 ?F (-2 ?C) (0.473 fledglings/pair) than when it was below 28 ?F (0.183 fledglings/pair). The number of pairs breeding in each of these situations was small, ranging from 2 to 7 [157]. The similarity in California spotted owl reproductive trends over large portions of the southern Sierra Nevada also suggests weather is an influential factor [185]. There are no data on the effects of weather on Mexican spotted owl reproduction.

Survival and occupancy rates of northern spotted owls may also be affected by weather. Northern spotted owl survival in Douglas-fir/hardwood and mixed-conifer forests of northwestern California was detrimentally affected by cold, wet springs [64]. In coniferous forests of the eastern Cascade Range of Washington, precipitation was inversely related to site occupancy [104]. There is no information on the effect of weather on California or Mexican spotted owl survival or site occupancy.

Good quality habitat likely buffers the effects of weather. For example, data from Douglas-fir/hardwood and mixed-conifer forests of northwestern California suggest that decreases in northern spotted owl survival associated with cold, wet weather were more gradual in landscapes with features considered high quality compared to landscapes with low-quality features [64]. The habitat features associated with nest sites and the negative impact of precipitation on California spotted owl reproduction led researchers to speculate that high canopy cover and foliage volumes could reduce throughfall precipitation and wind penetration at nest sites [157].

Weather may also influence spotted owl's habitat selection. In Saguaro National Park, the average daytime temperature at Mexican spotted owl roosts was significantly cooler than the surrounding ambient temperature (P<0.05), with an average difference between roost and ambient temperatures of 5.1 ?F (2.9 ?C) [205]. In northern Arizona, nesting sites had significantly (P<0.001) lower temperatures and were above 95 ?F (>35.2 ?C) less often than randomly-selected areas. Cooler sites were associated with increased canopy cover (P=0.001) [70]. According to a review, canyons occupied by Mexican spotted owls in Zion National Park had higher humidity than canyons where owls were not detected, and roost sites in Utah occurred in canyons with lower temperatures than randomly-selected canyons [78]. North and others [157] suggest that the characteristically high foliage volume at California spotted owl nest sites in oak woodland and mixed-conifer forests of the southern Sierra Nevada may reflect selection for microhabitats that provide the most cover in inclement weather.

Northern spotted owls in western Oregon roosted higher in the canopy in cold wet weather than in warm or hot weather [62]. However, relationships between roost sites and climatic variables were weak in mixed-conifer and Douglas-fir forests in southwestern Oregon [42]. Summaries of the role of northern spotted owl thermoregulation in the selection of old-growth habitat are included in [41,83].

Succession: Northern spotted owls are strongly associated with mature and old-growth forests [23,42,62,100,105,161,164,180]. In Douglas-fir/hardwood, mixed-conifer, and Oregon white oak forests of California, 500-acre (200 ha) plots centered on nesting and roosting sites contained significantly (P=0.003) more mature and old-growth habitat than random plots [100]. In mixed-evergreen forests of northwestern California, northern spotted owls preferentially selected (P≤0.005) foraging and roosting sites in mature or old-growth stands within home ranges [180]. In coniferous forests of the western Cascade Range of southwestern Washington, there was significantly (P<0.02) more forest 130 years old or older in 500-acre (200 ha) areas around nest sites compared to random sites [161]. In a western Oregon study, over 90% of roosting and nesting locations were in old-growth coniferous forests, and northern spotted owls foraged in these forests significantly (P<0.05) more than would be expected based on availability within their home ranges [62]. In coniferous forests in southwestern Oregon, 83% of northern spotted owls selected old-growth forests and used an average of 1.5 times the amount of old growth than would be expected based on availability in the landscape [42]. However, there was no difference in stand age between small nest and random plots in the eastern Cascade Range of Washington [32], and only 12% of nest stands on the eastern slope of the Cascade Range in Washington and Oregon were classified as old growth [53].

The use of younger stands by northern spotted owls is mixed. For instance, in coniferous forests of the western Cascade Range in southwestern Washington, northern spotted owls selected (P<0.02) stands less than 49 years old more than expected based their availability [161]. In the coastal redwood zone, forests more than 60 years old and those less than 7 years old were used in proportion to their availability, while stands from 31 to 60 years of age were used for nesting more (P≤0.039) than expected based on availability [59]. However, in mixed-evergreen and Klamath montane forests of northwestern California, pole timber and seedling-sapling stands less than about 35 years old were not used for nesting. Roosting individuals used the pole timber stands, but less (P<0.001) than would be expected based on availability [23]. In mixed-evergreen forests of northwestern California, summer foraging and roosting sites occurred significantly (P≤0.05) less often in young- to intermediate-aged stands than expected based on availability [180].

Greater amounts of mature and old-growth forests have been associated with improved northern spotted owl persistence and reproductive output in some areas. For instance, persistence on territories in southwestern Oregon was significantly (P<0.03) higher with increased amounts of forests more than 120 years old in the 8,870-acre (3,590 ha) area around the nest site [18]. The average number of fledglings per site was significantly (P<0.05) higher in 990-acre (400 ha) areas of western Washington, western Oregon, and northwestern California with more than 60% mature (>80 years) forest than in areas with 20% or less mature forest [19].

In other locations the relationship between forest age and northern spotted owl reproductive success has been ambiguous. In forests of the northern California Coast Ranges, greater reproductive success was negatively associated with clearcuts less than 6 years old at 2 scales, positively associated with 21- to 40-year-old forest at 3 scales, and negatively associated with 61- to 80-year-old forest at all 5 scales investigated [188]. See [188] for more detail regarding forest age class selection at different scales. In the eastern Cascade Range of Washington, the amount of late-successional forest (dominated by trees with >25-inch (64 cm) diameter at breast height (DBH)) within a 500-acre (200 ha) area of the nest was negatively associated with reproductive rate. Previously occupied territories that were not used in the last 3 years of the study had significantly less forest in seedling and sapling stages and significantly more forest dominated by pole-sized trees than territories occupied during this period (P<0.05) [104].

Northern spotted owl home range size may be influenced by the amount of old-growth habitat available. In mixed-conifer and Douglas-fir forests of southwestern Oregon, home range size was inversely correlated (r=-0.83) with the percentage of old-growth habitat. The amount of old growth used ranged from 1,330 to 2,360 acres (538-955 ha), which was less variable than the 1,920- to 8,980-acre (777-3,635 ha) home range sizes [43]. A review cites a 1984 Six Rivers National Forest publication for an association between home range size and amount of old growth [82]. The northern spotted owl recovery plan also includes citations for an inverse relationship between the amount of old-growth and home range size [158].

Although much less information is available on importance of old-growth habitats to California and Mexican spotted owls, data suggest that they select old growth and/or avoid young stands. Of oak-pine and riparian forests within California spotted owl home ranges in the Sierra National Forest, 91% were old growth. In Douglas-fir, ponderosa pine, and white fir dominated forests of the Sierra Nevada, early-successional stands including clearcuts, shrublands, and plantations were avoided by foraging individuals. Only 2% of telemetry locations occurred in these cover types, while they made up 30% of the available habitat. Home ranges were also comprised of significantly (P<0.001) less of these cover types than would be expected based on availability. Increased old-growth and mature tree basal areas were also characteristic of stands occupied by California spotted owls [36]. In mixed-conifer forest of the Sierra Nevada only 13% of the vegetation within California spotted owl home ranges was classed in the greater than 21-inch DBH category. The low reproductive rate in this area suggests that it was not providing high quality habitat [213]. In northern Arizona some foraging Mexican spotted owls selected old-growth mixed-conifer and ponderosa pine forests (generally >200 years old) more than expected based on availability within the home range, while managed forests in this area were avoided [71]. According to a literature review, continuous forests used by Mexican spotted owls are typically old-growth forests [83] and territories in Arizona usually contain mature trees, as well as other features associated with mature and old-growth habitats [46].

For information on the age of trees used by spotted owls for nesting and roosting, see those sections.

Tree characteristics: Spotted owls typically select areas with large trees associated with mature and old-growth stands. In coniferous forests of Tahoe National Forest, foraging California spotted owls used large (≥21 inch (53.2 cm) DBH) tree stands significantly (P<0.005) more than expected based on availability [36]. A summary of a California spotted owl habitat study in the Tahoe National Forest reports significantly more foraging sites in stands of large (20- to 35-inch (51-89 cm) DBH) trees than expected based on availability and significantly more foraging sites in these stands than in stands of medium-sized (11- to 20-inch (28-51 cm)) trees (P<0.01). An analysis of data from the national forests of the Sierra Nevada showed that California spotted owls nested in stands of medium to large (≥24-inch (61-cm DBH)) trees more than expected based on availability [85]. Several sources note Mexican spotted owls' use of large, mature trees [46,93,205], including roosting in areas with high densities of relatively large Douglas-fir and southwestern white pine (Pinus strobiformis) in coniferous forests of New Mexico [93]. Northern spotted owl high-use sites in coniferous forests of northwestern Washington had higher densities of trees greater than 31.5 inches (80 cm) DBH and higher foliage volumes than rarely used sites (P<0.1) [156]. In low to mid-elevation conifer forests of northwestern California, nest stands had significantly more (P<0.005) conifers greater than 35 inches (90 cm) DBH than would be expected based on availability [114]. In the eastern Cascade Range of Washington, the average height of the dominant canopy in small nest plots was significantly (P≤0.02) taller than canopy height on random plots within nest stands [32]. The average size of trees greater than 39 inches (100 cm) DBH was significantly (P<0.001) larger at nest sites than random sites within home ranges in coniferous forests in Oregon and Washington. However, trees greater than 21 inches (>53.4 cm) DBH did not occur at significantly greater densities or basal areas on nest sites than on random sites. Trees from 25 to 126 feet (7.6-38.3 m) in height were significantly more dense on nesting sites than random sites (P<0.1), while density of trees in the tallest (>176 feet (53.7 m)) categories were similar on nesting and random sites [91].

Northern and California spotted owl may select habitats dominated by intermediate-sized trees in some areas. In the eastern Cascade Range of Washington, the average density of intermediate-sized (14 to 24 inches (35-60 cm) DBH) Douglas-fir trees in small northern spotted owl nest plots was significantly (P=0.03) greater than that in random plots within nest stands [32]. Basal area of trees 21 inches (53.3 cm) DBH or smaller was significantly (P<0.001) greater on small nest plots than on random sites in coniferous forests in Oregon and Washington [91]. California spotted owl home ranges contained significantly (P<0.001) more forests comprised predominantly of 11- to 21-inch (27.0-53.1 cm) DBH trees than expected based on availability in coniferous habitats within Tahoe National Forest [36]. Although the amount of variation in tree DBH was similar on northern spotted owl nest sites and random sites in coniferous forests of Oregon and Washington [91], tree size variability may be important to spotted owls in some portions of their range [55,78,114,192].

Areas with more large trees may provide higher quality habitat. In previously logged forests in the northern Coast Ranges of California, northern spotted owls with greatest reproductive success had territories with a greater density of remnant large trees than less successful individuals. These differences were significant at the 120-acre (50 ha, P=0.042) and 208-acre (114 ha, P=0.052) scales [188]. In contrast, areas with dense small trees may be associated with lower site fidelity. In the eastern Cascade Range in Washington, areas with few 5- to 7-inch (13-19 cm) DBH trees were used more often than those with more of these pole-sized trees. Abandoned territories in this study area contained significantly (P=0.049) more pole-sized stands than occupied territories [104]. In conifer forests of southern Sierra Nevada, high foliage volume above the nest was related to California spotted owl nest success [157]. In the Lassen National Forest, stands dominated by large (>24 inches (61 cm) DBH) trees were associated with greater occupancy and apparent survival, and those areas with more large remnant trees were associated with increased nest success. Nest areas dominated by small trees had lower site occupancy and reproductive output. However, pairs in stands dominated by medium-sized (12-24 inches (30-61 cm) DBH) trees had higher nest success than those in stands dominated by large trees [24].

Although a common feature of northern spotted owl nest plots, mistletoe infestation in nest plots was similar to infestation in unoccupied stands [53] and nest stands [32] in the Cascade Range. For instance, there was no significant difference (P>0.3) in mistletoe infestation rating between small northern spotted owl nest plots and nest stands, although 84% of nest plots were infested [32]. Although the mistletoe infestation rate was not investigated, medium to large (>11 inches (27.5 cm) DBH) trees at California spotted owl foraging locations had significantly (P<0.001) lower vigor than those at random locations in coniferous habitats of Tahoe National Forest [36].

For information on the characteristics of trees used by spotted owls for nesting and roosting, see those sections.

Canopy cover/Tree density: Areas used by spotted owls typically have greater than 40% canopy cover and often have more than 70% canopy cover [18,20,24,55,104,184,197,216]. Despite variation in tree height and DBH, canopy cover varied little, ranging from 88% to 95% on small nest plots in grand fir forests of eastern Washington. Areas containing 90% of telemetry locations typically had greater than 40% canopy cover in this region [55]. Review of several studies found that northern spotted owl nesting and roosting sites in the California Klamath and Coast regions had more than 80% cover [18]. Canopy cover within 82 feet (25 m) of California spotted owl nest sites in foothill riparian and oak woodlands in the southern Sierra Nevada averaged 86% [184]; and in white fir-mixed-conifer forests in northeastern California, canopy cover was typically greater than 80% in a similar-sized area around nest sites [24]. According to a review, cover of vegetation above 7 feet is typically more than 70% at California spotted owl nest sites, although canopy cover as low as 30% to 40% has been observed at higher elevations [197]. Although a review notes the occurrence of Mexican spotted owls in sparsely vegetated habitats [83], other reviews state that they typically occupy stands with more than 40% [20] or 60% canopy cover [216]. In ponderosa pine-Gambel oak forests of Arizona, canopy cover was typically greater than 40% with only 2 roost stands having canopy cover from 25% to 40%. In addition, 75% of roost stands had more than 60% canopy cover [74].

Spotted owls may select areas with high canopy cover [36,70,71,80,91,205]. In coniferous forests in Oregon and Washington, canopy cover in small plots near northern spotted owl nests was significantly (P<0.095) greater than at random plots within spotted owl home ranges. However, when data were not pooled across study areas, only nest sites in the Olympic and Cascade regions had significantly (P<0.004) greater canopy cover than random plots [91]. Six California spotted owls in coniferous habitats of the Tahoe National Forest used stands with more than 40% canopy cover significantly more, and stands with less than 40% canopy cover significantly less than expected based on availability (P<0.005). In addition, California spotted owl home ranges were comprised of significantly (P<0.02) more forest with more than 70% canopy cover than expected [36]. California spotted owls in coniferous forests in the Sierra Nevada consistently selected areas with high canopy cover [213]. According to a review, roost sites in the Sierra Nevada and in southern California had significantly higher canopy cover than random sites. However, canopy cover at nest sites was not significantly higher than random sites in all study areas [85]. In northern Arizona, Mexican spotted owl nest stands averaged 75.2% canopy cover, which was significantly (P<0.001) higher than the 53.8% average canopy cover in random stands [70]. In a ponderosa pine-Gambel oak forest in Arizona, stands with more than 60% canopy cover were used for roosting and foraging in both the breeding and nonbreeding seasons more than expected based on availability. Stands used for roosting had significantly (P≤0.03) greater canopy cover than stands that were not used for roosting [76]. Other studies that have found greater canopy cover in areas used by Mexican spotted owls than random areas are summarized in [20,78].

Spotted owls may also select habitats with high live tree basal areas. The average live tree basal area on small plots at and around northern spotted owl nests in the eastern Cascade Range of Washington was significantly (P=0.09) greater than that of random plots within nest stands [32]. In forests of the northern Coast Ranges in California, there was a significantly greater proportion of the >69 m?/ha basal area category within 17 acres (7 ha), and significantly lower proportions of the less than 23 m?/ha basal area class within 17 acres (7 ha) and 124 acres (50 ha) of owl sites compared to random sites (P<0.05) [188]. A review notes that California spotted owls in the Sierra Nevada and in southern California nested and roosted in areas with greater average conifer and total live basal areas than random locations [85]. In northern Arizona, basal area in Mexican spotted owl nest stands averaged 37.9 m?/ha, which was significantly (P<0.001) greater than the average 25.4 m?/ha in random stands [70]. According to a Mexican spotted owl habitat analysis in a review, the basal area at Mexican spotted owl nests in the Basin and Range East region was significantly (P=0.0121) greater than in random forest stands, and in the upper Gila region the basal area at nests was significantly (P<0.0001) greater than within nest and random forest stands [78]. In Douglas-fir/hardwood forests of northwestern California, the smaller northern spotted owl males foraged in stands with higher tree density than stands used by the larger females [180].

Investigations of the impact of canopy cover and basal area on spotted owl habitat have found different relationships [24,33,99,121,188,210]. In forests in the northern Coast Ranges of California, activity centers of northern spotted owl pairs in the upper 50th percentile of reproductive success had higher proportions of the 23 to 69 m?/ha basal area classes and lower proportions of the >69 m?/ha basal area category. Many of these relationships were significant (P≤0.053) at scales from 124 to 983 acres (50-398 ha) [188]. In young forests of mainly western hemlock and Sitka spruce on the Olympic peninsula, sites used multiple times had an average canopy cover of 85.6%. Although high, this was significantly (P=0.03) lower than canopy cover on unused sites or sites within northern spotted owls' home ranges that had been used once [33]. In coniferous forests in the Sierra Nevada, there were weak but significant correlations (0.29<r<0.37, P≤0.04) between canopy cover and California spotted owl reproduction. The amount of forests with 0 to 39% canopy cover was negatively correlated, and the amount of forest with more than 40% cover was positively correlated with reproductive output at the 3 scales investigated (178-1,063 acres (72 ha-430 ha)). There was typically about 10% more habitat with canopy cover >50% on sites that consistently produced young compared to unproductive sites [99]. This difference was due to increased frequency of nesting pairs with increasing canopy cover, not higher reproduction by nesting pairs [121]. Geographic variability in spotted owl prey species may partially explain these differences and is discussed further in the foraging habitat section.

For information on canopy cover and tree density at spotted owl nesting and roosting locations, see those sections.

Cover type/Dominant tree species: Although some northern and California spotted owls select and avoid certain cover types, trends related to cover type or species composition are not consistent. The average Douglas-fir basal area of small plots on and around nests was significantly (P=0.02) greater, and the average basal area of lodgepole pine (Pinus contorta), Engelmann spruce (Picea engelmannii), and western white pine (Pinus monticola) was significantly (P=0.02) less than in random plots within nest stands in the eastern Cascade Range of Washington [32]. One study found that northern spotted owls in areas of northwestern California avoided hardwood stands [23,51]. However, Folliard and others [58] found no evidence for selection of cover types in northwestern California, with northern spotted owl nests occurring in cover types ranging from purely coniferous forests to those dominated by hardwoods. Northern spotted owls in northwestern California selected forests with understories comprised of oak [114]. Community type was rarely included in models for predicting spotted owl presence in old-growth forests of Olympic National Park. This indicates that habitat structure is more influential than species composition in northern spotted owl habitat selection [140]. According to a review of California spotted owl habitat, nests of California spotted owl were observed in mixed-conifer stands more than expected based on availability in national forests of the Sierra Nevada [85]. However, in middle elevation mixed-conifer forest of the central Sierra Nevada, cover types were not significantly different between 1,129-acre (457 ha) plots on California spotted owl activity centers and random areas. The authors also summarize evidence that California spotted owls occur in a wider range of cover types than northern spotted owls [143].

Mexican spotted owls apparently respond more consistently to cover type than northern and California spotted owls. Roost sites in southern New Mexico occurred primarily in mixed-conifer forest, and owls selected mixed-conifer stands for roosting in all but 1 season/site combination [77]. Selection of mixed-conifer habitats was also observed in the Black Range and San Mateo Mountains of New Mexico [93] and in the upper Gila Mountains region of Arizona [78]. In Arizona, the majority of foraging and roosting occurred in ponderosa pine-Gambel oak forests. Foraging and roosting stands in pine-oak areas also had generally higher Gambel oak density and basal areas than unused stands [76]. Similar trends have been observed in the Upper Gila Recovery Unit [76] and in the Black Range and San Mateo Mountains [93]. Gambel oak's contribution to canopy cover, canopy layering, and prey habitat in mixed-conifer-hardwood and pine-oak communities likely influences its selection by Mexican spotted owls [76]. A review notes selection of white fir in the Basin and Range East region and several firs (Abies spp.) in Utah. There was significantly more Douglas-fir, Gambel oak, and limber pine (Pinus flexilis) on roost sites than random sites in the upper Gila Mountains region [78]. Roosting Mexican spotted owls may have selected areas with greater southwestern white pine densities in southwestern New Mexico [93]. Ponderosa pine and pinyon-juniper woodland were avoided by Mexican spotted owl in southern New Mexico [77,93]. Ponderosa pine was also more abundant in random stands than in nest stands in the Upper Gila Mountains and Basin and Range East recovery units, according to a review [78]. In southern New Mexico, Mexican spotted owls had greater survival, increased fecundity, and smaller home ranges in a mesic area dominated by mixed-conifer forest than in a xeric area dominated by ponderosa pine forests and twoneedle pinyon-alligator juniper woodlands [77].

For information on tree species used for spotted owl nesting and roosting, see those sections.

Canopy layering: Northern spotted owls are strongly associated with multilayered forests. An uneven-aged, multiple-layered canopy is consistently included in descriptions of northern spotted owl habitat [42,55,62,114,134,180]. Increased canopy layering was a significant predictor of northern spotted owl presence in Olympic National Park [140]. In coniferous forests of northwestern Washington, the height class diversity, a measure of canopy layering, was significantly (P<0.1) greater in stands with more than 10% of telemetry locations compared to stands that were used less often [156]. In coniferous forests on the eastern slope of the Cascade Range in Oregon and Washington, nest sites had more sapling, pole, and large trees, while unoccupied stands within 2,953 feet (900 m) of the nest site had more medium-sized trees. This indicates a more layered canopy in occupied stands [53]. However, the average number of canopy layers in small northern spotted owl nest plots was not significantly different from that on random sites within nest stands in the eastern Cascade Range of Washington [32].

California spotted owls also used multilayered forests. Foraging sites of 6 California spotted owls had significantly (P<0.001) more vegetation layers than random locations in coniferous stands in the Tahoe National Forest [36]. In foothill riparian and oak woodlands in the southern Sierra Nevada, California spotted owl nest sites occurred in areas with multilayered canopies. The subcanopy was a dense (743 stems/acre) layer of trees less than 5 inches (13 cm) DBH that averaged 13 feet (4 m) tall. The major canopy layer was moderately dense (129 stems/acre) and was comprised of trees 5 to 30 inches (13-76 cm) DBH and 31 to 60 feet (9-18 m) tall. The upper canopy was sparse (1 stem/acre) and comprised of trees 31 to 60 inch (79-152 cm) DBH and 68 to 102 feet (21-31 m) tall [184]. Reviews also note California spotted owl's use of forests with multilayered canopies [85,197] and suggest the possible importance of vertical layers in providing perching sites, various microclimates, and higher prey species diversity [197]. See the FEIS review of tanoak (Lithocarpus densiflorus) for a concise summary of the importance of a hardwood subcanopy for northern and California spotted owls.

The Mexican spotted owl recovery plan [78,192] and a literature review of their habitat requirements [216] note the use of multilayered forests by Mexican spotted owls. In Saguaro National Park, small plots on Mexican spotted owl roost sites had significantly (P=0.049) more vegetation layers than random plots [205].

Understory: Areas occupied by spotted owls have varied understories. Tall shrub cover was significantly (P<0.098) greater on small plots around nest sites than on random sites within home ranges in coniferous forests in Oregon and Washington [91]. Anecdotal evidence suggests that shrubs in a shelterwood cut provided northern spotted owl foraging habitat [82]. However, shrub and herb cover were lower in areas frequently used for foraging and roosting than on infrequently used sites in mixed-evergreen forests of northwestern California [180]. In forests in the western Cascade Range, stands frequently used by northern spotted owls for foraging had lower herb and shrub cover than random stands in the same age class [105]. In foothill riparian and oak woodlands in the southern Sierra Nevada, California spotted owl nest sites had little vegetative ground cover, with an average of 20% small shrub cover and 21% grass cover. An average of 60% of the forest floor was covered by small litter [184]. A review of Mexican spotted owl habitat data notes that Rinkevich and Gutierrez (1996) found higher litter cover on roost sites than on random sites in southern Utah [78]. A review of the impacts of ungulate grazing on native species of the southwest suggests that a variable shrub layer provides both habitat for prey species and foraging habitat for Mexican spotted owls [216].

Snags: In some areas, northern spotted owls select habitats with more snags. For instance, in coniferous forests Oregon and Washington, basal area of relatively sturdy snags was significantly (P<0.001) greater in small plots near nests than in random sites within home ranges [91]. In young coniferous forests on the Olympic Peninsula in Washington, sites used repeatedly by northern spotted owls had significantly (P=0.0007) more snags greater than 20 inches (51 cm) DBH than random or single-use sites [33]. Similarly, in coniferous forest of Oregon, snags larger than 16 inches (40 cm) DBH were more abundant in stands used frequently for foraging than in random sites in forests of the same age class [105]. Snag volume was also greater (P<0.1) on northern spotted owl high use sites than on less often used sites in coniferous forests of northwestern Washington. Stands with snag basal areas less than 142.1 m?/ha were typically used less frequently for foraging [156]. Basal area of highly decayed snags (P<0.001) and density of small snags (P=0.08) were greater on small nest plots in the eastern Cascade Range of Washington than on random plots within nest stands. However, basal area of hard snags was significantly lower (P<0.01) on nest plots. Densities of larger snag size classes, and basal areas of snags in other decay classes were similar on nest sites and random sites within nest stands [32]. In coniferous forests of Washington and Oregon, the abundance of snags was similar on nest sites and unoccupied stands within a 0.6-mile (0.9 km) radius [53]. In old-growth coniferous forests of Olympic National Park, snag diameter was a significant predictor of owl presence. However, the relationship changed with study area, with larger snags associated with owl presence on the eastern side of the Bailey Range, and smaller snags associated with owl presence on the western side [140].

California and Mexican spotted owls may select habitats with abundant snags. According to a review, snag basal area at California spotted owl nest and roost sites was significantly (P<0.05) greater than at random sites [85]. Foraging locations in coniferous forests of the north-central Sierra Nevada also had significantly (P<0.001) greater snag basal area than random locations [36]. In foothill riparian and oak woodlands in the southern Sierra Nevada, California spotted owl nest sites averaged 14 snags/acre [184]. A review and analysis of habitat data tentatively recommends snag basal areas in Sierra mixed-conifer forests of 15 to 30 ft?/acre in foraging habitat and 30 to 55 ft?/acre in roosting and nesting habitat. It also suggests that snags greater than 15 inches (38 cm) DBH comprise 7 to 17 ft?/acre of foraging stands and 20 to 30 ft?/acre of nesting and roosting stands [85]. Mexican spotted owl roosting and foraging sites in coniferous forests of Arizona had significantly (P<0.001) greater snag densities and basal areas than random sites. See the table below for details [71]. A review reports significantly (P<0.0001) greater average snag densities in areas close to Mexican spotted owl nest sites in the Upper Gila Mountains Recovery Unit in Arizona and New Mexico. In small nest plots, snag density averaged 63.9/ha. Nest stands averaged 44.0 snags/ha and random stands averaged 17.6 snags/ha. Snag basal area was also significantly (P=0.0003) higher in nest stands than in random stands. Occupied canyons in Zion National Park also had higher snag basal areas than canyons where Mexican spotted owls were not detected [78].

Coarse woody debris: Northern spotted owl may select habitats with more coarse woody debris in some areas, possibly due to the apparent dependence of northern flying squirrels (Glaucomys sabrinus) and other spotted owl prey species on coarse woody debris for cover and truffle production [197]. In coniferous forests in Oregon and Washington, the volume of highly decayed logs was significantly (P≤0.025) greater in small plots near northern spotted owl nests than in random sites within home ranges [91]. The volume of downed logs with diameters greater than 20 inches (50 cm) was significantly (P=0.0002) greater on nesting and frequently used foraging sites than on random sites in coniferous forest in the western Cascade Range in Oregon [105]. However, in the eastern Cascade Range of Washington, the average volume of coarse woody debris in small nest plots was similar to that in random plots within nest stands [32]. Log abundance was also similar in nest stands and unoccupied stands within a 0.6-mile (0.9 km) radius of nest sites in coniferous forests on the eastern slope of the Cascade Range in Washington and Oregon [53].

Coarse woody debris is likely an important feature of Mexican and California spotted owl habitats. In coniferous forests of Arizona there were significantly (P<0.001) more logs with greater than 12-inch (30.5 cm) diameters and lengths of 10 or more feet (3 m) on sites used by 8 Mexican spotted owls for roosting and foraging than on random sites [71]. According to a review, the average amount of coarse woody debris was 97.8 m?/ha at nest sites and 94.7 m?/ha in nest stands in the Upper Gila Mountains Recovery Unit of Arizona and New Mexico. This was significantly greater (P=0.0006) than the average of 54.6 m?/ha in random stands. Nest sites in the Basin and Range East Recovery Unit in New Mexico also had higher (P=0.0061) log volumes than random stands [78]. Six California spotted owls selected 0.1-acre (0.04 ha) foraging plots with significantly (P<0.001) more cover of coarse woody debris than on random sites in the Tahoe National Forest [36]. Winter roost sites in the Eldorado National Forest also had significantly (P<0.05) more coarse woody debris than random sites, according to a literature review and analysis [85]. Another review states that coarse woody debris is more influential in coniferous forests than in riparian/hardwood habitats, due to the role of hypogeous fungi in supporting spotted owl prey in coniferous forests [197]. In foothill riparian and oak woodlands in the southern Sierra Nevada, the amount of coarse woody debris greater than 10 inches (25 cm) in diameter ranged from 156 to 331 ft?/acre [184]. A review and analysis of habitat data gives tentative recommendations for coarse woody debris density in Sierran mixed-conifer forests of 10 to 15 tons/acre in nesting, roosting, and foraging stands. It also recommends that coarse woody debris be >11 inches in diameter. The lower end of observed coarse woody debris densities and larger pieces of coarse woody debris were recommended to reduce the risk of fire in spotted owl habitats [85]. See the Habitat-related Fire Effects and Fire Ecology sections for more detailed information on the risk of fire in spotted owl habitat.

Foraging Habitat: Although similar to nesting and roosting habitat requirements, foraging habitat requirements are likely less strict. Like nesting and roosting sites, foraging areas are generally older [43,76,180], have higher canopy covers [36,85], greater tree densities [36,78,85], and more snags and coarse woody debris [78,85,105] than random sites. In the Coast Ranges of southern Oregon, northern spotted owls foraged in forests older than expected [43]. In northwestern California, foraging sites were older than sites that were infrequently used [180]. However, California [85] and northern [105] spotted owls may use younger forests for foraging than for nesting or roosting. For instance, stands as young as 27 years old were used by foraging northern spotted owls in coniferous forests of the western Cascade Range in Oregon [105]. Although Mexican spotted owls foraged in old stands and stands with more than 60% canopy cover in ponderosa pine-Gambel oak forests in Arizona, canopy cover on foraging sites was lower than on roosting sites [76]. California spotted owls did not select areas with greater canopy cover at the patch scale (≥5 acres, 2 ha) as consistently for foraging as they did for roosting in the Sierra Nevada [213]. Log volume and snag basal area and density were similar on foraging and roosting sites on sites in Arizona and New Mexico, but stands with less than 60 ft?/acre basal area, less than 25% canopy cover, and very high Gambel oak densities were used for foraging and not for breeding-season roosts [74]. Mexican spotted owl foraging sites also differed from roosting sites in study areas near Flagstaff and Alpine, Arizona, with large coarse woody debris, density of snags, tree density, and canopy cover significantly (P<0.001) higher on roosting sites [71]. Some sources suggest that foraging habitat has more open area under the canopy to allow for spotted owl flight [81,92,197]. Increased habitat heterogeneity (See Landscape/Scale Effects) may also be important to foraging spotted owls by providing more varied prey [59,65,200].

Predominant prey (See Food Habits) is likely to contribute to differences in spotted owl habitat selection across their range, since spotted owl prey species occur in a variety of habitats. For instance, in coniferous forests of southwestern Oregon and the Olympic Peninsula in Washington, flying squirrel densities in old-growth forests were typically about twice those in younger stands [42], while dusky-footed woodrats (Neotoma fuscipes) in northwestern California tend to occur in early-successional shrublands [171]. Young forest stands did not positively affect California spotted owl reproduction or site occupancy in a white fir-mixed-conifer study area on the Lassen National Forest [24]. According to a personal communication cited in a review, northern flying squirrel was the major prey item in this area [197]. In contrast, in areas where dusky-footed woodrats are a substantial component of the diet, such as in some areas of northwestern California [17,59], some interspersion of younger or more open stands with mature and old-growth forests is likely beneficial [59,64,188]. Northern spotted owl home range size in Douglas-fir and mixed-conifer study areas in southwestern Oregon and northwestern California was inversely correlated with proportion of woodrats (Neotoma spp.) in the diet (r=-0.8, P<0.005). It is suggested that the larger mass and tendency of woodrats to occur at higher densities than flying squirrels makes for more efficient foraging and allows for smaller home ranges [211]. The 1,700-ha area of old growth used by northern spotted owls in western hemlock forests of Washington was much greater than the 500-ha area of old growth used in mixed-conifer and Douglas-fir forests of Oregon, possibly due to the greater prey biomass density in the Oregon study area (388 g/ha) than the Washington study area (61 g/ha) [42]. Reviews (e.g. [14,207]) and more detailed information (e.g., [25,42,124,171,202]) on spotted owl prey species' availability and the habitats they select are available.

Nonbreeding habitat: Although habitat of northern and California spotted owls during the nonbreeding season is generally similar to breeding-season habitat, some differences have been reported. A review reports that northern spotted owl displays less consistent habitat selection in the nonbreeding than in the breeding season [18]. According to another review, wintering habitats of northern spotted owls in the Klamath physiographic region tend to have more and larger hardwoods, such as tanoak. California spotted owls use habitats with similar canopy closure in the breeding and nonbreeding seasons, but nonbreeding habitat is generally lower in elevation and less structurally complex [83]. Preliminary data cited in a review suggest wintering habitat occurs at lower elevations than breeding habitat in the Sierra Nevada [120].

Mexican spotted owls winter in lower elevation habitats that are more open and shrubby than breeding season habitats, according to reviews [78,83]. Based on a review of radio-telemetry data from studies in Arizona, New Mexico, Colorado, and Utah, roost stands used by Mexican spotted owls in the nonbreeding season had less basal area of live trees and hardwoods and lower canopy cover than breeding season roost stands. In addition, there were more forests with more than 60% canopy cover in breeding season ranges than in winter ranges [73]. In ponderosa pine-Gambel oak forests of Arizona, roosting sites occurred in more variable habitats in the nonbreeding season than in the breeding season. Nonbreeding season roost sites occurred in areas with basal areas less than 60 ft?/acres, less than 25% canopy cover, and very high oak densities, while these habitats were not used for roosting during the breeding season [74].

Dispersal habitat: Spotted owls may disperse through vegetation that is more open than typical habitat [60,78,83,93,129,206]. Forsman and others [60] and reviews [47,83] note northern spotted owl dispersal through cover types generally considered unsuitable habitat, including relatively open areas. For instance, dispersal of northern spotted owls through fragmented landscapes of Washington and Oregon was noted in a review [129]. Another review on the status and trends of northern spotted owls and their habitat describes northern spotted owl dispersal habitat as forests with average tree diameters greater than 11 inches (28 cm), greater than 40% canopy cover, and open areas under the canopy for flight [47]. Mexican spotted owls dispersed through pinyon-juniper woodlands, mountain shrubland, desert scrub, and subalpine and mixed-conifer forests in southern Utah [206]. In southwestern New Mexico, 2 Mexican spotted owls used juniper savanna and grasslands during dispersal [93].

Landscape/Scale effects: Northern spotted owls apparently select large habitat patches. On Washington's Olympic Peninsula, the average area of habitat patches in 8,038-acre (3,253-ha ) circles centered on pair locations was significantly (P<0.01) larger than in circles centered on random sites [126]. In Douglas-fir forests of northern California, frequency of occurrence increased as stand size increased (P<0.1) from <25 acres (10 ha) to 52 to 247 acres (21-100 ha). Frequency of occurrence was also positively associated with continuous amount of hardwoods adjacent to the nest stand (P<0.05) [169]. In a model based on data from Douglas-fir/hardwood and mixed-conifer forests of California, survival was positively associated with large core habitat patches in an area of approximately 400 acres (160 ha) around territory centers. However, reproduction was negatively associated with the area of core habitat patches [64].

Less is known about California and Mexican spotted owls' relationships to patch size. In mixed-conifer forests of the central Sierra Nevada, 1,129-acre (457 ha) areas used by California spotted owls were not comprised of significantly (P=0.547) larger habitat patches than random areas [143]. However, in a model of California spotted owl dynamics and reserve design, persistence was greater when reserved areas were arranged in fewer larger patches than when they occurred in more smaller patches [9]. Nest stands in study areas throughout California averaged 100 acres (40.5 ha) [85]. A review suggests that during winter, smaller forest patches may be used than in the breeding season [83]. According to a literature review and panel of experts convened to investigate effects of grazing on native species of the southwest, Mexican spotted owls can use "small patches of dense nesting habitat" [216].

Although the results are mixed, spotted owls may be negatively impacted by fragmentation and isolation. This has most often been observed at scales from 500 (200 ha) to several thousand acres. In Douglas-fir/hardwood and mixed-conifer forests of northwestern California, northern spotted owl nest sites were significantly (P<0.01) less fragmented than random plots at approximately 500-acre (200 ha) and 1,100-acre (450 ha) scales. At approximately 2,000- to 10,100-acre (800-4,100 ha) scales, fragmentation on northern spotted owl nest sites and random sites were not significantly (P>0.05) different [100]. A similar pattern was observed in Douglas-fir forests of northern California, with no significant associations between northern spotted owl frequency and percent clearcut or total edge at large (2,500 acre (1,000 ha)) scales. Northern spotted owl frequency also had no significant association with distance to a clearcut or total length of edge within the plot at small scales (25 acre (10 ha)). However, at the stand scale, northern spotted owl frequency was negatively associated with a measure of the percentage of the stand's perimeter that bordered clearcuts (P<0.1) [169]. Although several measures of fragmentation did not differ (P≥0.14) between northern spotted owl and random areas at a large scale in the Olympic Peninsula in Washington, a measure of northern spotted owl habitat isolation was significantly (P<0.01) lower in 8,038-acre (3,253-ha) circles centered on pair locations than in random circles [126]. Effects at large scales were observed in mixed-conifer and Douglas-fir landscapes in southwestern Oregon. Northern spotted owls in fragmented areas had lower densities, less home range overlap between members of the same pair, higher home range overlap between members of neighboring pairs, and greater incidence of mate changes than owls in study areas with clumped habitat [42]. Results from a model of California spotted owl dynamics and reserve design suggest that a lightly harvested matrix between areas reserved for spotted owls would result in longer persistence times than a heavily harvested matrix, due to quicker colonization of unoccupied habitat [9]. A review [192] notes that persistence of Mexican spotted owl populations may also be beneficially impacted by higher connectivity between patches, although another review [216] concludes habitat connectivity is not crucial for Mexican spotted owls based on dispersal of more than 19 to 25 miles (30-40 km) through unsuitable habitat.

In some areas, northern and Mexican spotted owls may benefit from habitat heterogeneity, while available information suggests California spotted owls in the Sierra Nevada are negatively associated with habitat heterogeneity. In the coastal redwood zone, approximately 500-acre (200 ha) plots centered on northern spotted owl nesting sites had more (P=0.003) total edge than random plots. The positive association appeared related to the length of boundaries between different cover types or between age classes of the same cover type [59]. Modeling the role of habitat in survival and reproduction of northern spotted owls in Douglas-fir/hardwoods and mixed-conifer forests of northwestern California suggests that owls in approximately 400-acre (160 ha) areas with more edges between spotted owl habitat and other cover types have higher reproduction and, to an extent, higher survival. Overall, high quality spotted owl habitat has greater apparent heterogeneity than low-quality habitat [64]. However, northern spotted owl in the same study area had marginally (P=0.058) less seral stage heterogeneity than random plots at a scale of approximately 500 acres (200 ha) [100]. A review states that Mexican spotted owls require heterogeneous habitat with edge and notes the use of mixed-conifer forest edges and montane meadows for foraging [216]. In contrast, areas used by California spotted owl had significantly (P=0.0002) lower habitat heterogeneity than random locations at the 1,129-acre (457 ha) scale in middle elevation mixed-conifer forest of the central Sierra Nevada [143].

Differences in the abundance and diversity of prey species across the spotted owl's range are often cited to account for the variation in area, edge, and heterogeneity of the habitat used [42,59,64,211]. For more information on the role of prey in habitat requirements see the Foraging habitat section. Differences in landscape character across their range may also explain some of the variation in spotted owl habitat use [18].

Spotted owls may be more selective of habitat in areas closer to their nest sites. In mixed-conifer and ponderosa pine-Gambel oak forests of north-central Arizona, Mexican spotted owls were most selective within about 500 acres (200 ha) surrounding the nest site [80]. In California's Klamath physiographic region, differences in habitat selected by northern spotted owls and those available at random were greatest at the 200-ha scale compared to the 500- and 900-ha scales. However, sample sizes were larger at the 200-ha scale [210].

Landscape-level requirements have led to recommendations to reserve closely spaced clusters of spotted owl habitat [116,129]. Modeling the response of northern spotted owls to reserves of varying size and spacing led to the conclusion that patches should be large enough to support 20 to 25 territories. Above this size, connectivity and extent of the reserve are just as important as patch size to the probability of population persistence [116]. The importance of connectivity for dispersal and mate-finding was noted by [9,154]. Arrangement of spotted owl habitat using an optimization technique was investigated by [94]. Several studies suggest that reserves be placed in a pattern that minimizes the risk of fire [15,38] or allows for mitigation of fuel reduction effects [167]. Minimizing the risk of fire in northern spotted owl habitat, including landscape-scale recommendations, is discussed in detail in the Habitat-related Fire Effects and Fire Ecology sections.

Home range/Density: The following home range sizes are calculated using a variety of field and statistical methods.

Spotted owl home ranges are generally large, but sizes are variable. The average home range size of northern spotted owl pairs varies from 1,030 acres (417 ha) in coniferous forests of Oregon [105] to 14,169 acres (5,734 ha) on Washington's Olympic Peninsula [126]. In riparian hardwood forests of the Sierra National Forest, California spotted owl had comparatively small home ranges, varying from 661 to 985 acres (267-399 ha), while those in mixed pine, white fir, and California red fir forests of the Lassen National Forest had home ranges varying from 7,061 to 12,473 acres (2,857-5,048 ha) [213]. A review notes median California spotted owl pair home range sizes of up to 18,706 acres (7,570 ha) [83]. A Mexican spotted owl review includes individual home range estimates from 645 acres (261 ha) in the upper Gila Mountains to 3,672 acres (1,487 ha) on the Colorado Plateau [78]. Pair home range estimates ranged from 2,548 acres (1,031 ha) in Arizona to 2,780 (1,125 ha) in New Mexico [83]. In some cases, Mexican spotted owls can spend a substantial portion of their time in a small portion of their home range [76,206]. For example, in riparian areas, pinyon-juniper, and mixed-conifer woodlands of southern Utah, 70% of radio locations occurred within an area averaging 689 acres (279 ha), which is less than one-third of the 2,179-acre (882 ha) area that was occupied by 95% of radio locations [206].

Spotted owls typically have smaller home ranges in the breeding season than in the nonbreeding season [43,62,73,76,206]. For example, in the western hemlock zone of the southern Oregon Coast Ranges, average northern spotted owl breeding season home range was 1,497 acres (606 ha), compared to an average nonbreeding season home range size of 3,509 acres (1,420 ha) [43]. Radio-telemetry data on Mexican spotted owls from studies in Arizona, New Mexico, Colorado, and Utah resulted in estimates of breeding season home ranges that varied from 563 to 1,250 acres (228-506 ha), while nonbreeding season home range estimates varied from 902 to 2,540 acres (365-1,028 ha) [73]. In a ponderosa pine-Gambel oak forest of Arizona, nonbreeding home range size were significantly (P=0.008) larger than breeding home ranges [76]. However, in the Sierra National Forest there was an interaction between cover type and season on California spotted owl territory size, such that nonbreeding territories were larger than breeding territories in mixed-conifer forest but smaller in oak/pine woodland [213].

Other sources of variability in home range size include habitat requirements and prey availability. Spotted owls likely require a certain amount of old-growth forests (see Succession). A literature review states that a pair of northern spotted owls uses an average of 2,000 to 2,500 acres of old-growth forests [41]. In areas where this resource is clumped and abundant, spotted owl home ranges are generally smaller. For example, in coniferous forests in southwestern Oregon, northern spotted owl home range size varied from 1,320 acres (533 ha) in clumped mixed-conifer forest to 7,190 acres (2,908 ha) in fragmented Douglas-fir forest [42]. Carey and others [43] as well as reviews [82,158] note the inverse relationship between home range sizes and the amount of old growth within them. Spotted owls also require less area in certain cover types [77,211,213]. For instance, Mexican spotted owl home ranges in the Sacramento Mountains of New Mexico dominated by mixed-conifer forests were significantly (P<0.04) smaller than those dominated by ponderosa pine forests and pinyon-juniper woodland. In addition, home range size was inversely related (P≤0.003) to the amount of mixed-conifer forest in the home range [77]. The variation between different habitat types may be related to the availability of spotted owl prey [42,158]. For instance, the low numbers and inconsistent availability of prey on Washington's Olympic peninsula was suggested as a cause for the large northern spotted owl home ranges in this region [158]. See the Foraging habitat section for more information on the role of spotted owl prey species in habitat use.

Female spotted owls may have larger home ranges than males. In mixed-evergreen forests of northwestern California, female northern spotted owl summer home ranges averaged 1,329 acres (538 ha), which was larger than the average male home range of 835 acres (338 ha) [180]. In pine-oak forest in Arizona, female Mexican spotted owls had considerably larger (P=0.073) home ranges than males [76]. However, there was no difference in the size of male and female California spotted owl home ranges in mixed-conifer and oak-pine woodlands of the Sierra National Forest and mixed pine, red and white fir in the northern Sierra Nevada [213].

The amount of overlap between members of pairs and between adjacent owls varies with season. In the western hemlock zone of the southern Oregon Coast Ranges, home range overlap of northern spotted owl pairs was 74% to 97% during the breeding season and 64% to 91% during the nonbreeding season [43]. In the Coast and Cascade ranges in western Oregon home range overlap averaged 68% between members of a pair and 12% for individuals in neighboring territories [62]. Home range overlap between members of California spotted owl pairs in coniferous forests of the Tahoe National Forest was 47% to 63% [36].

Spotted owl densities also vary with habitat and location. In mixed-conifer and Douglas-fir forests in southwestern Oregon, density of resident northern spotted owl pairs ranged from 0.046 pairs/km? in fragmented Douglas-fir forest to 0.190 pairs/km? in clumped mixed-conifer habitat [42]. In coastal redwood forests, Douglas-fir forests, and oak woodlands of northwestern California, average density of northern spotted owls was 0.209 owls/km?, but varied from 0.092 to 0.351 owls/km? across subregions. Ecological densities, defined as owls per area of suitable habitat, differed significantly (P<0.001) across the subregions and varied from 0.373 to 1.049 owls/km? [51]. According to a review of distribution and abundance information in the Mexican spotted owl recovery plan, Mexican spotted owl densities averaged 0.275 owls/km? in mixed-conifer forests, 0.08 owls/km? in pine forests, and 0.022 owls/km? in pinyon-juniper woodlands of the Sacramento Mountains New Mexico [201].

Nesting sites: Spotted owls do not build their own nests. They rely on sites such as trees and snags with cavities or broken tops, and platforms associated with abandoned raptor or squirrel nests, witches' brooms (caused by mistletoe infection) and debris accumulations [62,78,91,114,157,184]. Large, old trees are most often used by spotted owls for nesting. Species used as nest trees vary with region and subspecies. Several studies indicate that tree cavities are most commonly used for nesting by spotted owls, while the extent of platform use varies (see table below). In coniferous forests in Oregon, 60% to 93% of nests were in trees with broken tops. Additionally, broken-topped trees (>21 inches (53.3 cm) DBH

Northern spotted owl juvenile at a nest. Oregon Fish and Wildlife Service image.

Spotted owls typically nest in old trees in mature and old-growth forests (see Succession for more details). Sixty-five percent of northern spotted owl nests sites in coniferous forests of Oregon were in trees greater than 120 years old [105]. On 2 sites in the Coast and Cascade Ranges in western Oregon, 90% of nest sites were in unmanaged old-growth forests, 4% were in mature forests, and 6% were in late-successional forests (70-80 years) with 5 or fewer residual old-growth trees per hectare [62]. In low- to mid-elevation coniferous forests of northwestern California, the minimum nest tree age averaged 288 years, with a range of 57 to 688 years [114]. In coniferous forests in the Cascade Range of southwestern Washington, northern spotted owl site centers, such as the nest tree or locations of fledged young, did not occur in stands less than 49 years old, and 31% were in stands greater than 180 years old [161]. Most species of nest trees used by nesting California spotted owls in oak woodland and coniferous forests of the southern Sierra Nevada averaged more than 227 years of age [157]. A review notes that Ruess (1995) found 4 of 11 Mexican spotted owl nest sites in trees more than 115 years old, while this age class only comprised 0.5% of trees in the study area [78]. Moen and Gutierrez [143] and reviews [78,85] note the occurrence of California and Mexican spotted owl nests in large trees.

Selection of nest tree species varies with spotted owl subspecies and location. In Oregon, California, and most of Washington, more than 80% of northern spotted owl nests occurred in Douglas-fir [91,105,114]. However, on the Olympic Peninsula in Washington, where Douglas-fir was not present in all stands, northern spotted owl nests occurred with about equal frequency in Douglas-fir, western hemlock, and western redcedar [91]. A review states that California spotted owl's use of nest trees seemed related to availability, with Douglas-fir used most often in northern Sierra Nevada coniferous forests, and bigcone Douglas-fir, Jeffrey pine, and live oak (Quercus chrysolepis and/or Q. agrifolia) used primarily in southern California coniferous forests [85]. California spotted owls nested most frequently in pines and firs in white fir-mixed-conifer stands in the Lassen National Forest [24]. In foothill riparian and oak woodlands in the southern Sierra Nevada, nests occurred in California sycamore (Platanus racemosa, 5%), ponderosa pine (5%), and several oaks (88%) including interior live oak (Q. wislizenii, 34%), California black oak (Q. kelloggii, 22%), and canyon live oak (20%) [184]. In oak woodland and coniferous forests of the southern Sierra Nevada, California spotted owls selected giant sequoia (Sequoiadendron giganteum) and California black oak trees, with nests occurring in these species significantly (P<0.05) more often than their relative abundance [157]. According to a literature review of Mexican spotted owl habitat associations in Arizona and New Mexico, 50% of nests were in Douglas-fir, 20% in Gambel oak, and 19% in white fir. A study in ponderosa pine-Gambel oak communities found 6 of 11 nests in Gambel oak and the rest in ponderosa pine, while in the Tularosa Mountains of New Mexico, 78% of nests were in Douglas-fir, 11% were in white fir, 7% were in ponderosa pine, and 4% were in southwestern white pine [78].

Characteristics of nest trees and nest heights are displayed in the following table.

Some studies have included information on other nest tree characteristics. For instance, in coniferous forests in western Oregon and Washington, 77% to 83% of nest trees had fire scars [91]. California spotted owl nest trees in oak woodlands had high foliage volumes (544 m?) for their size (x=26 inches (66 cm) DBH) [157].

Many of the habitat characteristics discussed in the Preferred Habitat section have been observed in the area immediately around nest sites. For instance, overhead cover at northern and California spotted owl nest sites is high [22,62,157]. In oak woodlands above-nest foliage volumes of >900 m? in a 0.12-acre (0.05-ha) plot around nest sites were associated with greater reproductive output [157]. Tree density was also higher near California spotted owl nest sites in the Sierra Nevada [22,143,157]. Snags [22] and coarse woody debris [105] are also more abundant near some spotted owl nest locations.

Some spotted owl nest sites are used repeatedly. In the eastern Cascade Range of Washington, some nests were used in multiple seasons, including use by more than 1 pair of northern spotted owls [138]. On 2 sites in the Coast and Cascade Ranges of western Oregon, 68% of nests were used in multiple seasons [62]. A review of the California spotted owl's biology and ecology notes that California spotted owls may use the same nest annually, rotate the nests used, or use new nests each year [197]. Nests in oak woodland and coniferous forests of the southern Sierra Nevada used in 3 or more seasons had reproductive outputs more than twice those of nests used only once [157]. Although nests can be used multiple times, the attrition rate may be high. Over the course of an 8-year study in the Coast and Cascade Ranges in western Oregon, 17% of nest sites deteriorated to the extent that they were no longer usable [62].

Roosting sites: Unless otherwise stated, plot size in this subsection is 0.1 acre (0.04 ha).

Characteristics of spotted owl roosting cover are similar to those of nesting cover. For example, in Douglas-fir/hardwood forests of northwestern California, northern spotted owl roost plots had significantly (P<0.05) greater canopy cover than infrequently used plots [180]. In coniferous forests of Washington and Oregon, 86% to 88% of roost sites were in old forests, which were dominated by trees greater than 39 inches (100 cm) DBH and included patches of younger forests less than 10 acres (4 ha) in size [42]. In the central Sierra Nevada, California spotted owl nesting and roosting plots had similar canopy cover, snag basal area, ground cover of vegetation and coarse woody debris, and basal area, size, and condition of trees with DBH 11 inches (27.5 cm) or greater [22]. California spotted owl selection for high canopy cover was more consistent for roosting sites than nesting sites, according to a review of data from the central Sierra Nevada and southern California [85]. In the central Sierra Nevada, California spotted owl roost plots occurred in mixed-conifer forest dominated by trees larger than 12-inches (30 cm) DBH significantly (P<0.001) more than random sites [143]. In coniferous forests of Arizona, Mexican spotted owl roost plots had greater (P≤0.05) canopy cover, coarse woody debris, and tree and snag densities than random plots [71]. In Saguaro National Park, roost plots had greater canopy cover, more trees, more vegetation layers, and greater basal areas than random plots. In addition, trees in the roost plots were taller and had larger DBH than those on random sites (P≤0.05) [205]. However, a review notes that Mexican spotted owl roost trees are generally smaller than nest trees [78]. Spotted owl roosting and nesting habitat are also similar at larger scales [18,100,197] (see Preferred Habitat).

Detailed information on roost trees and roost heights is limited. In mixed-conifer and Douglas-fir forests of southwestern Oregon, northern spotted owl roost sites averaged 45 feet (13.7 m) above ground and ranged from ground level to 256 feet (78 m). Roost trees averaged 24 inches (61 cm) DBH and 83 feet (26 m) tall. These roost sites averaged 69% overhead cover [42]. A review notes that Ruess (1995) found 3 of 9 Mexican spotted owl roost sites in trees more than 115 years old, while this age class comprised only 0.5% of trees in the study area. The same review cites Fletcher and Hollis (1994) for data demonstrating that roosts occur in a wider range of tree sizes than nests in the upper Gila Mountains. Nearly 9% of roosts were in trees less than 6 inches (15 cm) DBH, and only 14.3% were in trees greater than 24 inches (61 cm) DBH, while less than 2% of nests occurred in trees less than 6 inches DBH and almost 43% of nests occurred in trees that had greater than 24 inches DBH. [78].

Spotted owls often roost in Douglas-fir, but several species are used. In mixed-conifer and Douglas-fir forests of southwestern Oregon, 46% of northern spotted owl roost sites were in Douglas-fir. Thirteen percent of roosts occurred in western hemlock and 9% were in western redcedar. Northern spotted owls also roosted in bigleaf maple (Acer macrophyllum), incense-cedar (Calocedrus decurrens), grand fir, giant chinkapin (Chrysolepis chrysophylla), Pacific madrone (Arbutus menziesii), and canyon live oak [42]. In Saguaro National Park about 60% of Mexican spotted owl roosts were in Douglas-fir. Gambel oak, southwestern white pine, ponderosa pine, New Mexico locust (Robinia neomexicana), and a white pine snag were also used for roosting [205]. In the upper Gila Mountains, 54% of roosts were in Douglas-fir and 21% were in Gambel oak, according to a review [78].

FOOD HABITS:
According to reviews, spotted owls are nocturnal, sit-and-wait predators. They often hunt from a perch and swoop or pounce on prey. They also grab arboreal prey from tree boles and limbs. They do occasionally hunt during the day and will hawk prey such as insects or bats [42,134,197,200].

Diet may affect spotted owl's reproductive output. In California, the average prey size was significantly (P<0.01) larger during years that northern and California spotted owls bred (x=115 g) compared to nonbreeding years (x=75 g) [16]. Although mean prey biomass of nesting northern spotted owls was generally higher than that of nonnesting owls in coniferous habitats of the Olympic peninsula and eastern Cascade Range in Washington, the difference was only significant (P<0.05) in 2 of 21 territories [63]. Preliminary results from the southern Sierra Nevada suggest that reproduction of California spotted owls was not associated with cone crop, an index of prey species abundance [155]. Nest success in coniferous forest, oak woodland, and riparian deciduous habitats of the Sierra National Forest was not related to the proportion of biomass comprised of northern flying squirrel [148].

Although spotted owls' diet varies with location, the majority is typically comprised of 1 to a few mammalian species. Species taken most often are northern flying squirrels and woodrats, including dusky-footed, bushy-tailed, (N. cinerea), and Mexican woodrats (N. mexicana). Northern flying squirrels commonly comprise greater than 30% of the biomass of the diets of northern spotted owls [62,63] and California spotted owls in conifer forests of the Sierra Nevada [148]. Woodrats comprise the majority of the diet of all 3 subspecies, in at least some portion of their ranges. In mixed-conifer habitats of northwestern California [17] and the Klamath Mountains of Oregon [62], dusky-footed woodrats comprised more than 50% of northern spotted owl diets. Woodrats comprise the predominant portion of the biomass of California spotted owl [17,148] and Mexican spotted owl diets [25,69,205]. Reviews provide additional summaries on these trends [83,158,197].

In some portions of its range, substantial proportions of the spotted owl's diet is composed of several other mammal species. White-footed mice (Peromyscus spp.), such as deer mice (Peromyscus maniculatus), comprised 2% to 30% of the total prey in northern spotted owl diets [17,62] and 16.6% to 30.7% in California spotted owl diets [17,147]. This genus represents from 0.7% to 6% of the biomass of northern and California spotted owl diets [17,62,63,148]. White-footed mice generally occur more frequently and comprise a larger portion of the diet of Mexican spotted owls [25,69,205] than that of the other 2 subspecies, with percent biomass as high as 17.3% in diets of Mexican spotted owls in pine-oak forests of northern Arizona [25]. Reviews of brush mice and deer mice are available in FEIS. Pocket gophers (Thomomys spp.) comprised up to 16.2% of the biomass of northern spotted owl diets [62,63], 18.5% of California spotted owl diets [17,148], and 15.5% of Mexican spotted owl diets [25,69]. Although voles (Microtus, Clethrionomys, and Phenocomys spp.) comprised up to 31.0% of the total prey in northern spotted owl diets [62] and 37.6% in Mexican spotted owl diets [69], percentage of biomass of voles in spotted owl diets is generally less than 5% [25,62,63,69,148]. However, at 2 sites in Arizona, voles made up 15% and 16.1% of the biomass of Mexican spotted owl diets (Microtus spp) [69]. In Washington snowshoe hares (Lepus americanus) comprised up to 16.3% of the biomass of northern spotted owl diets [63].  On one study area in Oregon 24.9% of prey biomass was snowshoe hares and brush rabbits (Sylvilagus bachmani), and 6.2% of prey biomass in the Klamath Mountains was brush rabbits [62]. Rabbits (Sylvilagus spp.) comprised up to 29.6% of the biomass of Mexican spotted owl diets, although values in the 7% to 8% range are more common [25,69]. Red tree voles (Arborimus longicaudus) represented as much as 10.3% of the biomass of northern spotted owl diets in Douglas-fir and western hemlock forests of Oregon [62] and averaged 4% in the Coast Ranges of northwestern California [17]. For more detail on variation in the use of red tree voles across the northern spotted owl's range in Oregon see [61]. Red tree voles do not occur within the range of California or Mexican spotted owls, according to a review of ecosystem issues in the northern spotted owl recovery plan [14]. Although squirrels such as Tamiasciurus and Tamias do not typically comprise substantial portions of the diet [40], reviews note these and other diurnal species making up fairly high percentages (5-20%) of the California spotted owl's diet in some areas [83,120,197]. Northern and California spotted owls occasionally consume moles and shrews [62,148], and Mexican spotted owls infrequently eat bats [25,69].

Nonmammalian prey species include birds, amphibians, reptiles, and insects. Birds, such as smaller owls (Strigidae), jays (Corvidae), songbirds (Passeriformes), and woodpeckers (Picinae) typically comprise less than 10% of the biomass of the spotted owl's diet. Insects can occur at fairly high frequencies in owl pellets but typically make up a very small percentage of the biomass of spotted owl diets [17,25,63,69,148]. Amphibians and reptiles are rarely preyed upon [63,69,148].

The extent to which various taxa are eaten by spotted owls varies temporally. Both annual [63,69] and seasonal variation in the composition of spotted owl diets has been observed. For instance, pocket gophers, voles, insects, and rabbits are generally more common in diet of spotted owls in the summer than in the winter [62,63,147]. In coniferous forests of the Sierra National Forest, birds comprised 12.9% of the biomass of the California spotted owl's diet during the breeding season but only 4.6% during the nonbreeding season [148].

PREDATORS:
Birds and mammals, such as the fisher (Martes pennanti), are likely predators of eggs and young spotted owls ([60], review by [83]). Northern goshawks (Accipiter gentilis) and crows (Corvus spp.) may prey on juvenile spotted owls, while great horned owls (Bubo virginianus), red-tailed hawks (Buteo jamaicensis), and golden eagles (Aquila chrysaetos) are likely predators of both juvenile and adults ([60,62,101]; reviews by [83,192,197]). Great horned owls and barred owls likely compete with spotted owls for food and space in some areas [72,161]. Barred owls may have a negative effect on northern spotted owl survival and fecundity in some areas [13]. For a review of the effects of barred owl range expansion on northern spotted owls see [128]. For information on the possible role of predator and/or competitor avoidance in spotted owl habitat selection see [41,42].

MANAGEMENT CONSIDERATIONS:
Status: Many studies have found spotted owl populations in decline (population growth rate < 1.0), though many of these declines are not statistically significant [65,127,199]. For instance, estimates of northern spotted owl annual population growth rate were <1.0 in all but 1 of 14 study areas, but 95% confidence intervals included 1.0 in all but 4 populations [13]. A study of 3 California spotted owl populations in the southern Sierra Nevada found that all of them had growth rates significantly (P<0.05) lower than 1.0 [185]. The San Bernardino population is also apparently declining [215]. A 7-year study in Arizona and New Mexico found 2 Mexican spotted owl populations declined by 10% or more per year [173]. For meta-analyses of spotted owl population data see [29,65].

According to reviews, possible causes for spotted owl population declines include climate change, barred owl invasion, and habitat loss and degradation due to logging, grazing, and/or stand-replacement fire [134,152,199]. For more information on the possible effects of barred owl invasion see [86,161]. Although policies implemented in the early- to mid-1990s have apparently had a positive impact on habitat trends, it is uncertain how long the trends will continue [47,144,152,163]. A review of the listing process for the northern and Mexican spotted owls, as well as actions taken to conserve all 3 subspecies, is provided by [83]. For a discussion of the effects of long-term and large-scale challenges such as urbanization and climate change on spotted owl recovery see [196].

Owl population management: Spotted owl populations are sensitive to the survival of adults and may show delayed responses to habitat loss [115,151]. Improving adult survival would likely have a large positive impact on owl populations. However, typically high adult survival rates (see Life span) suggest that it would likely be easier to increase population growth by improving juvenile survival and reproduction [151]. Due to the delayed responses of owls [115,151] and the variable nature of reproductive success (see Reproductive Output), study lengths of more than 10 years are probably needed to determine the influence of management actions on spotted owl populations [185].

Forest management: Logging can have negative effects on spotted owls. Pair separation [42] and home range abandonment [62] have been observed after logging operations. Areas that had a history of logging were avoided by California spotted owls in the northern Sierra Nevada [212]. Mexican spotted owls used managed mixed-conifer and ponderosa pine forests of Arizona less than expected based on availability within the home range [71]. According to reviews on the effect of logging on songbirds of southwest ponderosa pine forests [57] and development of silvicultural practices in the Sierra Nevada [198], the loss of large trees and snags, species composition changes, and general simplification contribute to negative response of spotted owls to logging [57,198]. However, the percentage of northern spotted owl nest sites and random sites with logging histories were not significantly different in the eastern Cascade Range of Washington, and 46% of nest sites had experienced some degree of timber harvest from 15 to over 40 years previously [32].

Recommendations for mitigating the impacts of logging on spotted owls include harvesting at appropriate times and locations. For instance, a review of issues related to incorporating sensitive species in ecological restoration efforts recommends performing treatments from 1 September to 28 February to prevent disturbing breeding Mexican spotted owls [20]. There have been several recommendations to suspend logging at and around nest sites [20,76,80,107,188], including the recommendation to restrict logging that creates openings with less than 10% cover within a mile of Mexican spotted owl nest sites in mixed-conifer and ponderosa pine-Gambel oak forests of north-central Arizona [80]. Inclusion of refuge areas and corridors into harvest areas to increase connectivity of spotted owl habitats and minimize the effect of clearcutting on disturbance processes has also been suggested [66]. For landscape-scale information not directly related to timber harvesting see the Landscape/Scale Effects section.

Treating slash, monitoring effects of treatments on owl populations, and retaining large trees, snags, and coarse woody debris have also been suggested to reduce logging effects on spotted owls. Since greater exposure to sun and wind can dry the fuels generated by thinning and increase fire risk, Weatherspoon and others [203] conclude that slash reduction is critical. Slash treatment is also recommended by [11,108,136]. However, Agee and Edmonds [5] suggest that treatment of slash is only necessary on the most fire-prone sites. Monitoring the effects of habitat alterations on spotted owl populations in an adaptive management framework is also recommended [20,104,107,152,192]. Preservation of large trees, including those with defects, is often recommended to provide cover, nesting and roosting sites, and a source of snags [55,74,92,103,136,157]. Retaining snags [45,50,92,183] and coarse woody debris [92,183], especially large pieces [45,85], is also suggested to minimize the impacts of timber harvest on spotted owls. Other recommendations include extending harvest rotations [11,45,157], avoiding logging on lower slopes ([11] and see Topography), retaining at least 6 Douglas-fir trees per hectare in grand fir-dominated forests of eastern Washington [55], leaving clumps of trees for roosting northern spotted owls [92], and retaining [89] or improving tree species diversity [11]. For an example of uneven-aged management within the California spotted owl's range see [179].

Timber harvesting may be able to accelerate the development of spotted owl habitat [11,89,92,152]. Thinning stands can promote the growth of big trees with fissured bark and large crowns, as well as multilayered stands [89,165]. Modelling of various treatments found that heavy to moderate thinning that began earlier in the simulation and included long rotations would encourage the growth of stands with species and tree sizes similar to those on northern spotted owl nest sites [11]. Creation of small gaps in extensive coniferous forest stands may provide foraging habitat in the eastern Cascade Range of Washington [104]. In dense stands, thinning may increase prey abundance and improve the ability of spotted owls to locate and capture prey [25,92]. Folliard and others [59] recommend harvests that are distributed in space and time and that do not compromise possible nest sites. Reviews of spotted owl habitat development are available [162,187]. For information on monitoring the effectiveness of spotted owl habitat creation and maintenance efforts see [90].

Thinning techniques to reduce the risk of severe fire in spotted owl habitat include creation of fuel breaks and removal of fuel ladders, such as small trees [20,121,163,182,203]. The reduction of fuel loads by thinning fire-intolerant, small and medium-sized trees is one of several recommendations suggested for reducing fire risk in northern spotted owl habitat [163]. A review suggests that thinning trees less than 9 inches (23 cm) DBH outside of a 100-acre (40 ha) area around the nest site, treating less than 50% of the zone between 100 and 600 acres (40-240 ha) around the nest site, and performing treatments in the nonbreeding season could reduce the risk of severe fire while minimizing the impacts of thinning on Mexican spotted owls. Thinning mid-sized oaks after prescribed burning, monitoring of treatment effectiveness, and retaining large trees, snags, large logs, and a hardwood component are recommended by [20]. Patch and variable retention thinning that mimic mixed-severity fire are recommended for maintaining northern flying squirrel habitat while reducing the risk of large, high-severity fires [124]. Construction of fire breaks that are strategically located and shaded was recommended for reducing the risk of fire in the eastern Cascade Range of Washington and Oregon, the California Cascade Range, and the Klamath Mountains [182]. Modeling fuels in a mixed-severity fire regime in southwest Oregon found the amount of thinning required to substantially reduce fire risk was more than could be implemented with the regulations in place at the time of the analysis [167]. For more information on the risk of fire in spotted owl habitat see the Fire Ecology section, and for details of the use of prescribed burning to reduce fire risk and/or the severity of future wildfires in occupied and potential spotted owl habitat see the Habitat-related Fire Effects and Fire Management Considerations sections.

Reviews of forest management practices and effects [192,199,216] are available. Information on decision support including discussions of managing for multiple, possibly conflicting, uses can be found in [28,145,163]. Planning at the landscape scale can allow for the implementation of multiple and conflicting uses [35,88,163].

FIRE EFFECTS AND USE

• DIRECT FIRE EFFECTS ON ANIMALS
• HABITAT-RELATED FIRE EFFECTS
• FIRE MANAGEMENT CONSIDERATIONS

It is likely that nests and young are more vulnerable to fire-caused mortality than adults [123,132,160]. A juvenile northern spotted owl is believed to have been killed in the Davis Fire of 2003 in the Deschutes National Forest [193]. Fires during the nonbreeding season likely have less direct impact on spotted owls [20,131,132]. A review of issues related to considering sensitive species in ecological restoration planning recommends fuel reduction treatments in Mexican spotted owl habitat be carried out from 1 September to 28 February [20]. Literature reviews have used fire characteristics and life history of species to speculate on possible effects of fire on nesting success and bird populations [131,132,166]. For instance, spotted owl nests located higher in the canopy may provide young with protection from the direct effects of low-severity fire [131]. However, the lack of renesting after failed nest attempts and inconsistent yearly breeding (see Breeding Cycle) suggest that fires during the breeding season could reduce spotted owl reproduction [132,166]. According to a review of the direct effects of fire on animals, more uniform burns may have greater negative impacts than patchy burns on the survival of nestlings and fledglings [132].

Spotted owls may shift or move home ranges in response to fire. Northern spotted owls shifted home range on 2 territories affected by the Klickitat and Lakebed fires in southern Washington. Before the fires, 45% and 14% of radio-locations were outside the area that later burned. After the fires, 73% and 50% of radio-locations were in unburned areas. The investigators suggest that spotted owls did not entirely abandon the burned sites due to their strong site fidelity [21]. California spotted owls apparently abandoned their territories for lower-elevation, unburned vegetation after the 1977 Marble-cone fire [52]. Postfire home ranges of northern spotted owls using the 2002 Timbered Rock Burn were typically bigger than home ranges documented in this region of Oregon before the fire [12]. There were no significant changes to Mexican spotted owl home range size after 2 prescribed burns outside of their primary nesting and roosting habitat in Saguaro National Park, although small sample sizes and confounding factors limit the conclusions that can be drawn from this data [205].

HABITAT-RELATED FIRE EFFECTS:
Although several articles discuss fire, spotted owls, and their habitat, results should be interpreted with caution. As noted by a literature review summarizing songbird responses to fire in southwestern ponderosa pine forests [57], there are several limitations to many studies addressing bird response to fire. For instance, few studies compare demographic parameters of spotted owls between burned and unburned vegetation, which is necessary to separate effect of fire from other factors. In addition, most studies, including those that address spotted owl demographics in burned and unburned areas, are opportunistic, anecdotal, restricted in spatial or temporal scale, have small sample sizes, and/or include confounding factors [57].

Spotted owl occupancy of burned areas is variable. Of 21 spotted owls occurring in areas affected by wildfires greater than 1,334 acres (540 ha), 18 were resighted at least 1 year after the fire, and 16 of these remained on their breeding territory following the fire. These values for survival and site fidelity are similar to overall averages [27]. Of 56 confirmed California spotted owl sites in Yosemite National Park, 6 had burned in prescribed natural fire within 8 years before the survey [203]. Burned sites within 0.6 miles (1.0 km) of Mexican spotted owl nests in mixed-conifer, pine, and pine/oak forests of Arizona and New Mexico had occupancy rates of 70%, versus 84% occupancy rate on unburned sites, although the difference was statistically insignificant (P=0.075) [107]. Following the 1994 Hatchery Complex wildfires in the eastern Cascades of Washington, northern spotted owl occupancy was the lowest it had been in 4 years. There was also a significant (P<0.05) association between the amount of unburned spotted owl habitat near (0.5-mile (0.8 km)) activity centers and their occupancy status, with extensively burned areas being unoccupied and largely unburned areas being occupied by reproductive pairs [68].

Effects of fire on spotted owl reproduction are also variable. There are anecdotal reports of spotted owls breeding in territories with burned habitat. For instance, Mexican spotted owls nested in areas that had been moderately burned by the 1990 Clark Peak wildfire in southeastern Arizona [172]. Mexican spotted owls in the Coconino National Forest bred successfully in most years following prescribed burns that did not negatively influence habitat structure [176]. In a territory close to prescribed burns in ponderosa pine forest of Saguaro National Park in southern Arizona, Mexican spotted owls fledged 2 young in the year following burning [10]. In mixed-conifer, pine, and pine/oak forests of Arizona and New Mexico, 3 territories with successful nests were more than 50% burned. Overall, burned sites had lower reproductive success. This difference was not statistically significant (P=0.011), but sample size was small [107]. Reproductive northern spotted owl pairs were significantly (P<0.05) more likely to occupy 0.5-mile (0.8 km) radius areas around activity centers that had less burned habitat following the Hatchery Complex wildfire. Although reproduction was low in burned areas the year following the fire, reproduction was low in all territories that year. In addition, the following year's reproduction in burned areas was not much lower than that in unburned areas [68]. Of 7 spotted owl pairs in which both members were resighted after fire, 7 fledglings were produced by 4 pairs. This reproductive rate is higher than the overall average. The 3 individuals whose mates were not resighted were paired the following breeding season [27]. After the 2003 Clark Fire, detection of northern spotted owl × barred owl hybrids increased [190]. Given the strong influence of weather on spotted owl reproduction, conditions after a fire are likely to affect the postfire reproduction of spotted owl.

The extent of spotted owl use of burned areas for foraging is uncertain. In the Coast and Cascade Ranges in western Oregon, northern spotted owls rarely forage in areas burned up to 20 years previously [62]. A review [82] describes an anecdotal observation of a radio-marked northern spotted owl that stopped foraging after a prescribed burn in a shelterwood cut until brush started regenerating. However, another review describes a case of radio-collared northern spotted owls spending substantial amounts of time in "burned-over areas" of northern California, possibly due to the ease of prey capture (Taylor 1995, as cited by [160]). It has been suggested that increases in forage after prescribed burns may increase Mexican spotted owl prey abundance [176].

Individual prey species are likely to have different responses to fire, which differentially affect spotted owl behavior. A review of the biology of California spotted owl prey species [207] suggests that the likely reduction in tree density and coarse woody debris volume from fire would reduce habitat quality for northern flying squirrels. Although fire could increase abundance of shrubby vegetation used by some woodrats, the review notes they are subjected to fire-related mortality, and recolonization of burned areas can take time [207]. For instance, in southern California it took 4 to 6 years for white-footed mice, pocket mice, and woodrats to recover to their prefire levels [209]. Conversely, woodrats showed no reduction in survival or emigration and only a small reduction in reproduction shortly after a prescribed fire in an oak-woodland understory in southern California [122]. Some species, such as deer mice, have been shown to increase following fires [113,207]. FEIS reviews on a few spotted owl prey species, such as bushy-tailed woodrat, brush mouse, deer mouse, meadow vole, and snowshoe hare are available.

The overall effect of fire on spotted owl foraging efficiency and prey availability is uncertain. Possible benefits of fire include the creation of a habitat mosaic that could increase diversity of spotted owl prey [176,192]. A review on the effects of fire on animal populations speculates that the decline in available cover for prey species and possible increases in prey populations due to postfire increases in their forage could improve spotted owl foraging efficiency [131]. However, in mixed-conifer forests of the southern Sierra Nevada, prescribed fires had negligible effects on small mammal biomass [146]. The Mexican Spotted Owl Recovery Plan notes that the net effect of fire on Mexican spotted owl prey species is not clear, although the loss of the canopy could interfere with the spotted owl's perch and pounce hunting technique [192]. In southern Washington the body of a malnourished female northern spotted owl was found in a territory burned in a mixed-severity fire, suggesting a decline in prey and/or foraging ability [21].

Although high-severity fire tends to have negative impacts on spotted owls [21,68,81,97], they can apparently tolerate a limited amount of stand-replacement fire [12,27,107,203]. Instances of spotted owls occurring in territories burned extensively by stand-replacement fires have been reported for all three subspecies [27,203]. Northern spotted owls were observed using stands burned at stand-replacement severity in the 2002 Timbered Rock Fire [12]. In mixed-conifer, pine, and pine/oak forests of Arizona and New Mexico, successful production of fledglings occurred on three territories that experienced stand-replacement fire over 8%, 31%, and 32% of their area. Fire severity was not significantly associated with the breeding/occupancy of Mexican spotted owl territories [107].

Frequent fires are likely detrimental to spotted owls, as they would likely reduce spotted owl habitat features, such as multiple canopy layers and coarse woody debris. Of 3 survey areas in mixed-conifer, pine, and pine/oak forests of Arizona and New Mexico that burned multiple times within the 4 years prior to the study, 2 were occupied by single owls and one was occupied by a nonreproducing pair. On 31 unburned sites, 5 were unoccupied, 5 were occupied by single owls, 16 were occupied by pairs, and reproduction was confirmed on the other 5. Time since fire may have little impact on Mexican spotted owls in the short-term. Occupancy on sites burned the previous year was 75%, while 68% of those burned 2 to 4 years previously were occupied [107]. In a model of California spotted owl dynamics and reserve design, decreases in fire frequency and smaller lag times between fire and spotted owl recolonization resulted in improved spotted owl persistence, although mortality rates and reserve arrangement had larger influences [9]. Sheppard and Farnsworth [176] recommend not treating areas within the same management territory in consecutive years, while a review of fire ecology in the Pacific Northwest suggests that large treatments within designated conservation areas be separated by at least a decade [2].

Fire effects on stand structure and composition: Key features of spotted owl habitat such as high canopy cover, large trees, and multiple-layered canopies will likely be damaged or destroyed by high-severity fire and can take extended periods to recover. An average of 55% of the habitat within 6 northern spotted owl activity centers was lost within 1 year of the Hatchery Complex wildfires in the eastern Cascade Range of Washington. Suitable habitat in this area was defined as having >60% canopy cover, 2 or more canopy layers, and numerous snags and downed logs [68]. Although the effect on canopy cover may be comparatively short-lived [214], the recovery of a multiple-layered canopy and development of large trees can take decades or even centuries. Huff [96] found that the understory reinitiation phase, during which subcanopy layers develop, had begun in a coniferous forest on the Olympic Peninsula 181 years after the Mineral Creek fire. In a model of forest succession and response to silvicultural prescriptions, stands that develop without silvicultural treatments did not develop size distributions and species compositions similar to northern spotted owl nest sites within the 160-year timeframe [11]. Given the possible reduction in spotted owl habitat due to removal of subcanopy layers, Agee [2] suggests limiting the spatial and temporal distribution of underburning in designated conservation areas.

The effects of fire on spotted owls are likely influenced by habitat composition. Associations between the occupancy/breeding status and percentage of pine and percentage of mixed-conifer were significant (P<0.04) in Arizona and New Mexico. Spotted owls were typically absent from areas where pines comprised 38% to 85% of burned sites, and successful reproduction in burned sites was related to greater percentages of mixed-conifer habitat [107].

Spotted owl responses to postfire changes in understory cover are variable. In redwood and giant sequoia forests, fires may benefit owls by opening understories [123]. In habitats with a hardwood understory, species that are relatively abundant around spotted owl nest sites, such as Rocky Mountain maple (Acer glabrum) and Scouler willow (Salix scouleriana) tend to increase after thinning and burning [106]. Gambel oak may be promoted by treatments that thin the understory according to a review of literature on sensitive species and fuel reduction [20]. A review of fire effects on vegetation and birds of the Pacific Northwest notes that fires may benefit northern spotted owls in northern California in the long term by providing a mosaic of forest structures [97].

Fire consumes, alters, and creates snags used by nesting spotted owls and coarse woody debris used by spotted owl prey. In a Sierran mixed-conifer forest, thin-and-burn and burn-only treatments resulted in a decline in the amount of fine and coarse woody debris, a reduction in the average size of woody debris, a shift to less decayed woody debris, and an overall reduction in large snag frequency [103]. Almost half of ponderosa pine snags with DBH greater than 6 inches (15 cm) were seriously damaged or destroyed after moderately severe surface fires in southeastern Arizona [95]. In at least some cases, snag recruitment can be low after fire [170], and fire-created snags may be comparatively small [95,103] or short lived [119,159]. Longevity of snags created by fire is influenced by snag size, species, degree of decay, density, and occurrence of salvage logging [34,170]. Recruitment of large snags is dependent on the retention of large trees [103], and "loss of larger snags could take decades to recover," according to a synthesis of the effects of fuel reduction on wildlife in dry coniferous forests [162]. However, increases in snags and coarse woody debris after fire have been documented. For instance, a prescribed burn in ponderosa pine habitat in Arizona resulted in increased snag density and greater amounts of medium-sized ground fuel [67]. Reviews of the importance of woody debris to wildlife [31] and effects of fuel reduction on wildlife [162] note that charring of snags and coarse woody debris can alter their value to wildlife. Finch and others [57] summarize fire effects on snags and coarse woody debris.

Vulnerability of existing snags and coarse woody debris to fire depends on several factors. Size and age/decay class can influence the vulnerability of a snag [95,119,170]. Lower severity fires may result in greater declines in snag abundance, due to the loss of existing snags without comparable recruitment of new snags [67,165]. For instance, of 2 prescribed burns in ponderosa pine habitat in Arizona, the lower severity fire resulted in declines in snag density and amounts of surface fuels in all size classes, while the higher severity fire resulted in increased snag density and amounts of medium-sized surface fuel [67]. Although burning outside the breeding season is recommended to reduce impacts on spotted owl reproduction [20,176], burning when fuel moistures are high, such as in late winter or spring, can reduce the consumption of coarse woody debris and other litter [203].

Fire frequency and time since fire also influence coarse woody debris and snag characteristics. In Douglas-fir dominated forests of Washington and Oregon, sites that experienced reburns early in succession had the largest and most prolonged declines in the amount of coarse woody debris [181]. Time since fire is also a factor in snag and coarse woody debris recruitment [95,119,159,181,186]. In ponderosa pine forest of Arizona, volume of coarse woody debris was lower on recently (≤4 years) burned sites (2.99-4.85 Mg/ha) than on somewhat older (8-9 years) sites (5.85-10.03 Mg/ha). Amount of coarse woody debris was greatest in ponderosa pine forests of Arizona burned by wildfire about 8 years previously [159]. Studies mostly of pine (Pinus spp.) trees in the western United States found the majority of tree mortality occurred within 5 years of fire [95,119,186].

Landscape factors: The amount, arrangement, and connectivity of habitat remaining after fire may influence spotted owl's postfire response. For instance, the amount of remaining habitat within 0.6 mile (1 km) of northern spotted owl activity centers the eastern Cascade Range of Washington was significantly (P<0.05) correlated with their breeding and occupancy status [68]. Although there may be lags in the response, reductions in carrying capacity and habitat connectivity could affect spotted owl population persistence [9,15,115,116,151,154]. Fires close to the nest or in heavily-used foraging areas probably have greater negative impacts than fires in less used portions of the home range [107]. Smaller impacts of fire may also occur if the remaining habitat occurs in large patches and/or in a matrix of habitat that has not been greatly altered [9]. For a review of landscape considerations in relation to animal species responses to fire see [130].

In highly connected habitat, especially in the drier regions of the spotted owl's range, high-severity fire can become widespread, damaging spotted owl habitat over large areas. For example, a build up of fuels in the Klamath region may have contributed to the 2002 Biscuit Fire burning at stand-replacement severity over approximately 78,700 acres (31,900 ha). This represents approximately 75% of the loss to stand-replacement wildfire that occurred from 1994 to 2003 in forests dominated by medium (20-29.9 inches (51-76 cm)) and large (≥30.0 inches (76 cm)) trees [144]. A review of fire effects on vegetation and birds of the Pacific Northwest suggests that fires in areas with greater amounts and continuity of fuel may impact larger areas and have longer-lasting effects on spotted owls than they experienced historically [97]. Placing reserved habitat in areas that are relatively unlikely to burn is one method for reducing the risk of spotted owl habitat loss to fire [6,37,163]. Areas that may be less likely to burn include the lee sides of lakes and lava flows [6], riparian areas [39,68,163], slopes with northern aspects [39,163], headwalls at high elevations [39,163] and canyons used by Mexican spotted owls for nesting in Colorado [49]. However, with high fuel continuity there is a risk of fire spreading from adjacent areas into fire refugia [107,133]. Fuel treatments that are strategically located may decrease the likelihood of high-severity fire spreading into critical habitat and allow for safer and more effective fire suppression [5,68,168,177,192,203,204]. Reducing fuels in areas that are more likely to burn, such as south-facing slopes, may prevent stand-replacement fire from spreading into critical areas [5,68,192,203,204]. Placement of fire breaks in shaded areas between critical habitat zones has been recommended to compartmentalize fires [5,182].

Avoiding fuel treatment around spotted owl nesting and other high-use sites has been recommended for reducing the impacts of treatments on spotted owls [10,20,191]. The lack of long-term and causal data on all three subspecies led Bond and others [27] to conclude that systematic burning within territories is not currently justified. Agee and Edmonds [5] recommend that prescribed natural fire areas not include northern spotted owl designated conservation areas or a buffer zone surrounding them. A review suggests that no more than 50% of the 600-acre (243 ha) area around Mexican spotted owl nests should be treated at one time, and no treatment should occur within 100 acres around Mexican spotted owl nest sites [20]. Sheppard and Farnsworth [176] recommend limiting burning to 25% or less of Mexican spotted owls' core area in a given year and maintaining at least 25% of this area in an unburned state.

Over the long-term the mosaic of habitats created by mixed-severity fire may provide some benefits to spotted owls, at least in certain portions of their range. Several studies have found that a high percentage of northern spotted owl nest sites have evidence of previous fires [32,53,91]. In the Klamath region, evidence of fire was found at nest sites significantly (P<0.028) more often than at random sites. The complex fire history of the region may have contributed to the diverse species composition and stand structure that occurs at northern spotted owl nest sites [91]. Northern spotted owl habitat quality was apparently higher in more heterogeneous regions of Douglas-fir/hardwood, mixed-conifer, and Oregon white oak forests in northwestern California [64]. For more information on the effects of habitat heterogeneity on spotted owls see the Landscape/Scale Effect section. In addition, patchy fires may improve spotted owl habitat by increasing the diversity of prey habitat (see Foraging Habitat), reducing the risk of fire in forests with historically low- or moderate-severity fire regimes [15,192], and increasing horizontal diversity of the habitat [192]. A review of fire effects on vegetation and birds of the Pacific Northwest suggests that more uniform fire severity reduces the structural diversity required by spotted owls [97].

Fire Ecology: Spotted owls occur in habitats with a wide range of fire regimes, from vegetation with long fire-return intervals such as Sitka spruce-western hemlock communities to cover types that typically experience low- to moderate-severity fires at much shorter intervals, such as mixed-conifer communities of northern California. Several sources note the variation in fire regimes in habitats occupied by the northern [1,5,53,97,178], California [189,195,203], and Mexican [176] spotted owl. The fire season in most northern and California spotted owl habitats occurs from mid or late summer to fall [111,178,195]. In the range of the Mexican spotted owl, lightning-ignited wildfires are typical during the summer monsoon season [107,109], while human-ignited fires can occur year round [109]. Summaries of fire regimes in the habitats of northern [1,2,42,97,123], California [2,183,203], and Mexican spotted owls [109,176] are available.

Throughout much of the spotted owl's range, fire regimes have been altered. For instance, in 2001 only 19.7% of northern spotted owl territories on the eastern slope of the Cascade Range in Washington were within or near the historic range of fire frequency, and 43.8% deviated from historic frequencies by multiple fire-return intervals. Nests in ponderosa pine-Douglas-fir forests were at the highest risk, with 89 of 93 northern spotted owl sites in forests that had missed one or more fire intervals [106]. The majority of the older forests in California Cascades, California Klamath, and Oregon Klamath have missed 2 or more fire-return intervals [144]. In eastern Washington, the average fire frequency before 1900 at six northern spotted owl nest sites was 13 years, while after 1900 four of these sites experienced no fire and the other two had fire frequencies of 20 and 18 years [55]. Longer fire-return intervals and smaller fires were also observed after the start of fire exclusion compared to earlier time periods in the central Oregon coast range [102].

These deviations from historic fire regimes are thought to result in higher fuel loads, which may increase the risk of uncharacteristically large and/or severe fires [4,46,54,121,142,144,192,198]. For instance, fire exclusion over the last 60 years is considered the major factor in the significant (P<0.05) increase in stem density from about 1932 to 1992 in the San Bernardino Mountains [142]. Examples of shifts toward more severe fires after years of fire exclusion are provided by [3].

Much of the spotted owl's habitat in the drier portions of its range may be at higher risk of stand-replacement fire than before European settlement [2,97,121]. This risk is of concern in the eastern Cascade and Klamath regions [5,106,133], the mixed-conifer forests of the Sierra Nevada [121,203], and ponderosa pine and mixed-conifer forests of the southwest [107,176,192]. For instance, in the eastern Cascade Range of Washington, the locations of 27 of 29 northern spotted owl nest sites had less than a 20% chance of acting as a fire refuge [68]. For summaries of the possible relationships between altered fire regimes, fuel accumulations, fire severity, and spotted owl habitat see [2,97,106,121].

Fire management recommendations for regions differ depending on their historic fire regime. In mesic and moist cover types with historically high-severity fire regimes, fuel treatments may not be effective when weather conditions are conducive to fire spread. In addition, fuel reduction treatments could increase fire risk by creating more open and drier sites and increasing fine fuels [4,48]. Examples of high severity fire burning through thinned stands and fire breaks are included in [48]. However, it has been suggested that in habitats with historically low- and mixed-severity fire regimes, fuel reduction could reduce fire severities [4]. For example, in conifer and mixed-conifer hardwood forests of the Klamath mountains in northwest California, piling and burning slash and then underburning reduced the occurrence of high-severity fire despite instances of severe fire weather, with less than 5% of fully treated stands and 65% of untreated stands subjected to stand-replacing fire [108].

Due to the likely reduction in spotted owl habitat quality from fuel reduction [2,124], balancing spotted owl habitat with fire hazard reduction has received considerable attention [6,35,61,98,121,125,163]. Modeling California spotted owl habitat in the southern Sierra Nevada suggests that surface and understory fuel reduction can be implemented without substantial negative impacts on California spotted owls [121]. In coniferous forests of Oregon, a strategically located fuel treatment that reduced crown density and surface and ladder fuels in 20% of the area outside of spotted owl habitat resulted in a 44% decrease in the probability of spotted owl habitat loss from fire [6]. In the eastern Cascade Range of Washington, retaining complex forest structures and minimizing wildfire risk were compatible at the landscape scale when 45% or less of the area was reserved in a late-successional stage [35]. However, modeling of fuels in a mixed-severity fire regime in southwest Oregon led to the conclusion that the extent of treatment necessary to appreciably reduce the risk of fire is unfeasible with policy constraints existing in 2005 [167]. For summaries on maintaining complex, late-successional habitats in fire prone areas see [163,175].

For discussions of the effects of climate change on fire risk in spotted owl habitat see [5,137].

The following table provides fire regime information on vegetation communities in which spotted owl may occur. The inclusion and omission of habitat types in the following list was based on spotted owl distribution information and the habitat characteristics and species composition of communities spotted owls are known to occupy throughout the year. There is not conclusive evidence that spotted owls occur in all the habitat types listed, and several of the habitats listed, especially the more open, shrubby vegetation types, are used only on a limited basis during dispersal or the nonbreeding season. See the Plant Communities section for more information on cover types used by spotted owls. Find further fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find Fire Regimes".

Fuel reduction treatments are often recommended for reducing the risk of stand-replacement fire in the drier regions of the spotted owl's distribution [5,107,121,133,176,192,203]. Prescribed fires can be used to reduce smaller woody material and ladder fuels while retaining larger coarse woody debris and snags [10,176,203]. Ten-hour timelag fuel moisture has been used as a metric for burning prescriptions [10,135]. Articles related to the implementation of prescribed fire in or near California spotted owl habitat, including discussion of obstacles such as air quality issues, are available [135,183]. The adaptive management paradigm is recommended to provide information on the response of spotted owls to fuel reduction and other habitat maintenance activities [20,104,107,152,192]. See the Forest management section for recommendations on the use of mechanical thinning to reduce the risk of fire in or near spotted owl habitat, as well as citations related to decision support.

Many of the recommendations for minimizing the loss of spotted owl habitat to stand-replacing wildfire focus on managing habitat at the landscape scale. Locating reserved habitats in areas that are unlikely to burn, such as northern aspects and riparian areas, could reduce the risk of large scale habitat loss to stand-replacing fire [6,68]. Placement of fuel treatments where decreased fuel continuity can prevent spread of high-severity fire is often recommended [5,68,182,192,203,204], as is placement of fuel reduction treatments outside spotted owl high-use areas [5,10,20,27,107,204]. See the Landscape factors section for more details on spatial recommendations for minimizing the risk of stand-replacement fire in spotted owl habitat.

In addition to spatial considerations, the impacts on spotted owls and their habitats from fuel-reduction prescribed fires can be mitigated by timing treatments appropriately and preserving large trees, snags, and coarse woody debris. It is recommended that prescribed fires or other fuel reduction treatments within or near owl habitat be conducted during the nonbreeding season, from early September to late January or February [20,176]. In addition, it has been recommended that fuel reduction treatments be performed relatively infrequently in and near areas occupied by spotted owls [2,176]. However, outside of these areas, reburning within 10 years has been recommended to maintain low fuel levels and to consume fuels created by earlier prescribed burns [103,135,203]. Raking debris away from large trees, snags, and coarse woody debris [20], burning in late winter or early spring when fuel moisture is high [203], and maintaining low fuel loads [174] have been recommended to reduce the impacts of prescribed burns on spotted owl habitat structure. Large (>11 inches (28 cm) diameter) coarse woody debris densities of 10 to 15 tons per acre may be reasonable amounts for both spotted owl habitat and acceptable fire risk [199,203]. A minimum density "equivalent to 10 ft²/acre of snags and downed logs" was recommended for mixed-conifer forests within the range of the Mexican spotted owl [176].

Some recommendations have been made for areas where severe fire has damaged or destroyed spotted owl habitat. In habitats such as live oak-bigcone Douglas-fir, recovery after fire can take decades [141]. Planting of bigcone Douglas-fir seedlings, removal of competing shrubs, and thinning of sprouting canyon live oaks could accelerate habitat recovery [203]. As in fuel reduction thinning, retaining any remnant trees, large snags, and coarse woody debris in postfire thinning operations may provide important habitat elements for spotted owls [20,24,45,83,92,188]. For more information on accelerating the development of spotted owl habitat see the Forest management section.

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