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Nucifraga columbiana

• INTRODUCTORY
• DISTRIBUTION AND OCCURRENCE
• BIOLOGICAL DATA AND HABITAT REQUIREMENTS
• FIRE EFFECTS AND USE
• APPENDIX: FIRE REGIME TABLE
• REFERENCES

INTRODUCTORY

	AUTHORSHIP AND CITATION FEIS ABBREVIATION COMMON NAMES TAXONOMY SYNONYMS ORDER CLASS FEDERAL LEGAL STATUS OTHER STATUS
©Dave Menke, US Fish and Wildlife Service.

DISTRIBUTION AND OCCURRENCE

• GENERAL DISTRIBUTION
• PLANT COMMUNITIES

Throughout much of their range, Clark's nutcrackers inhabit communities dominated by high-elevation pines (Pinus spp.). Preferred subalpine communities include those dominated by whitebark pine (P. albicaulis) and limber pine (P. flexilis) [71,137]. Rocky Mountain bristlecone pine (P. aristata), Great Basin bristlecone pine (P. longaeva), and foxtail pine (P. balfouriana) communities are upper subalpine and/or treeline communities also used by Clark's nutcrackers, especially where these small-seeded pines occur near whitebark pine or limber pine communities [75]. At low montane elevations, Clark's nutcrackers frequent Jeffrey pine (P. jeffreyi) [137], ponderosa pine (P. ponderosa), and Douglas-fir (Pseudotsuga menziesii) communities. In the southern part of their range in the Great Basin and portions of the Southwest, Clark's nutcrackers are found in pinyon-juniper (Pinus-Juniperus spp.) woodlands, where singleleaf pinyon (P. monophylla) and/or Colorado pinyon (P. edulis) comprise the pinyon component of the overstory [169]. Southwestern white pine (P. strobiformis) communities are used at moderate elevations in the Southern Rockies [121]. Other community types where Clark's nutcrackers may be found include 1) low-elevation grassland-treeline and grassland-shrubland ecotones; and 2) high-elevation treeline/alpine ecotones, where patches of high-elevation pines commonly develop a krummholz growth form [147]. Due to their nomadic nature, Clark's nutcrackers are widely distributed throughout montane habitats. In the Northern Rocky Mountains, Clark's nutcrackers occurred in a wide variety of cover types within conifer forest, including black cottonwood (Populus balsamifera subsp. trichocarpa) and/or quaking aspen (P. tremuloides) bottomlands, riparian shrublands, wetlands, and sagebrush (Artemisia spp.) [47].

BIOLOGICAL DATA AND HABITAT REQUIREMENTS

• LIFE HISTORY
• PREFERRED HABITAT
• COVER REQUIREMENTS
• FOOD HABITS
• PREDATORS
• MANAGEMENT CONSIDERATIONS

• Reproduction and longevity
• Annual cycle
• Molt
• Migration
• Irruptions and extralimital wandering

Breeding: Reproductive frequency is not well understood in the Clark's nutcracker. Although adults may breed on an annual basis, successful overwintering and reproduction probably occur only in years when sufficient seed stores are cached by both parents [90,147]. Seed caches provide the energy and nutrients necessary for early breeding in montane habitats, when temperatures are cold and alternate food sources are in short supply [169].

The mating system of the Clark’s nutcracker was not well described in the literature as of 2008. Clark’s nutcrackers are apparently monogamous and may have long-lasting pair bonds [147]. It is unknown when and how the pair bond forms. Clark’s nutcrackers begin breeding their second winter and likely breed every year thereafter unless there is a seed crop failure of a primary food source (see Irruptions and extralimital wandering). Clark’s nutcrackers produce only one brood per season [98].

The Clark’s nutcracker begins breeding activities unusually early for a montane passerine–often in late February or early March [98,160]. Freezing and near-freezing temperatures are common, and snow is often present at the time of nesting [98]. Stored pine seeds provide the energy and nutrients needed for breeding under such harsh conditions [38]. Early breeding helps to ensure adequate food for the winter. By the time the new seed crop is ripe in the fall, juveniles must be capable of making their own seed stores for the coming winter and spring; once juveniles are independent, adults cache their own food stores [160]. An extended period of growth and development before the onset of harsh weather may also improve juvenile winter survival in montane habitats [160,169].

Clark’s nutcrackers live in pairs outside the breeding season and are typically members of wintering flocks [98,160]. Pairs leave these flocks to breed in late winter. Although pair bonds are reinforced by courtship behavior at any time of the year [147], actual courtship activities are most frequent from February through April [98,160].

Nesting, incubation and fledging stages: Clark’s nutcrackers nest in trees, usually on montane sites, and are solitary nesters [34,160]. The nesting season typically extends from March to April [98]. Onset and duration of the nesting season probably vary from year to year in relation to elevation and local conditions [136]. Mewaldt [98] observed nest building as early as 10 March in western Montana. Tomback [136] described a late nesting attempt by Clark’s nutcrackers on 1 June at Tioga Lake in Yosemite National Park, California, but the pair had abandoned the nest by 20 June.

Clark’s nutcrackers build sturdy, well-insulated nests that moderate cold temperatures. The nest is a platform of woven twigs supporting a thick-walled, inner nest cup lined with fine material [98]. Nest construction usually lasts about 5 to 8 days, with the first egg laid 2 days later [98]. Eggs are very pale green to greenish-blue with brownish flecks [17]. As eggs are laid, the nest is covered to protect them from freezing [98]. Clutch size typically ranges from 2 to 5 eggs (x=3.06 eggs/clutch (SD 0.67), n=32) [147].

Although somewhat social throughout much of the year, Clark’s nutcrackers commonly exhibit territoriality around the nest. On a montane site in Montana, Mewaldt [98] estimated the size of one nesting territory at 2.1 acres (0.85 ha); the territory was enclosed within 7 boundary perches. On the same site, 3 active nests were found, all within 1,640 feet (500 m) of each other [98]. Territorial defense against conspecifics appears quite individualized. Some Clark's nutcrackers nesting in the Washington Cascade Range aggressively defended their nest site against other Clark's nutcrackers; others were very tolerant of neighbors, sometimes permitting individuals within several meters of the actual nest (Lorenz 2008, personal communication [84]). Other passerine species are generally tolerated inside the territory [98]. Although territories are probably held by the same pair for many years [147], no field data were available on this topic as of 2008. Most food is obtained from outside the territory [98].

Clark’s nutcrackers begin incubating when the last egg is laid, and incubation lasts 18 days [98]. Both female and male Clark’s nutcrackers incubate [98]. The male has a brood patch as well developed as the female’s; in most passerines a brood patch occurs only in females [147]. Shared incubation and brooding free both parents to recover seed caches to feed themselves and nestlings. Hatching of altricial young is nearly synchronous [147]. The nestling period lasts approximately 20 days [98].

Clark’s nutcracker fledglings have an exceptionally long period of parental care compared to most passerines. Although Clark’s nutcrackers fledge in April and May [147], they are fed by parents until mid-July or even late August, when they are approximately 13 to 14 weeks old. Juveniles continue to depend on parents for most of their food because the cached pine seeds that provide the bulk of the juvenile diet can only be retrieved by the adults that cached them the previous year [169,172] (see Foraging and caching).

Life span and survivorship: Because Clark’s nutcrackers wander widely in response to food shortages, counts of previously banded birds are not good indicators of survivorship [147]. In Rocky Mountain National Park in Colorado, Tomback [147] reported at least 36% of birds banded in 1985 returned one year later (n=78); of total birds banded in 1985 and 1986 (n=145), at least 3 birds lived until 1994. The maximum recorded life span of the Clark’s nutcracker is 17 years 5 months [25].

Annual cycle: The annual cycle of the Clark’s nutcracker is closely tied to its specialized year-round diet of fresh and stored pine seeds [137]. As a general pattern, harvest and storage of pine seeds occurs in late summer and fall when seeds are produced; recovery of seed caches occurs most frequently during the winter and spring, when other food is limited (see Foraging and caching).

Details of the annual cycle of Clark's nutcrackers may differ among geographical regions. Studies by Tomback [137] in the Sierra Nevada and Lorenz [85] in the Washington Cascade Range exemplify patterns from 2 markedly different habitats.

Sierra Nevada: Tomback [137] followed the annual cycle of Clark's nutcrackers for 4 years in the vicinity of Yosemite National Park. On these sites, Clark's nutcrackers moved between discrete elevational zones. The majority of Clark's nutcrackers overwintered and reproduced on low-elevation sites within the Jeffrey pine–pinyon-juniper–sagebrush ecotone; they spent the summer and much of the fall at subalpine elevations harvesting and caching whitebark pine seeds (see Migration) [137].

Cascade Range: Lorenz [85] tracked radio-tagged Clark's nutcrackers for several years in the Washington Cascade Range. On these sites, segments of the local Clark's nutcracker population exhibited different annual cycles according to their migratory status (see Landscape use). She grouped Clark's nutcrackers into 2 categories: 1) residents—individuals that maintain a year round home range where all food caches are concentrated and 2) emigrants—individuals that range over large areas in order find sufficient food and forage and cache opportunistically. Resident Clark's nutcrackers demonstrated a stable annual cycle characterized by fidelity to a year-round, 120-acre (500 ha) home range that contained the year's food caches; individuals remained there to overwinter and breed, living off previously cached seeds. Individuals only traveled outside the home range in autumn in order to harvest the year's seed crop. Although residents ranged up to 20 miles (32 km) in search of harvest trees, individuals always transported seeds back to the home range for caching. During 2 years of the study, most resident Clark's nutcrackers harvested whitebark pine seeds early in the season and shifted seed harvesting to lower-elevation ponderosa pine and Douglas-fir stands in mid-September (14 September 2006 and 21 September 2007) [85].

The emigrant portion of the local population showed a less predictable lifestyle. These individuals showed no fidelity to a year-round home range. They wandered over large areas throughout the entire year, exploiting areas of abundant seed production for short periods before moving on to new areas of concentrated seed sources. Emigrants placed caches opportunistically in stands scattered throughout an estimated 150 km², using a wide range of habitats and elevations. In all likelihood, individuals did not retrieve their own seed caches [85].

Molt: Molting (replacement) of feathers in the Clark’s nutcracker is closely tied to the harvest and transport of pine seeds. Most passerines begin molting immediately following spring breeding; during this time feathers are totally replaced within a matter of weeks, and flying abilities are generally compromised. Clark's nutcrackers, however, molt over a prolonged period, which ensures maximum flight capabilities throughout the year. The annual molt usually begins in March, when adults are beginning to breed, and may last 8 to 9 months. Some birds replace feathers in every month except February [17,99]. Since Clark's nutcrackers must be strong flyers to transport heavy loads of pouched seeds, both adults and juveniles grow new flight feathers (primaries) before the onset of seed harvest [99]. Adults begin replacing primaries during March and are finished by late August. Juveniles develop flight feathers from April through July; molting of body feathers occurs as early as July and continues until January [99].

Migration: Unlike other passerines, Clark’s nutcrackers do not migrate latitudinally, but remain at montane elevations all year long. However, irruptions outside the normal range occur during years of simultaneous cone crop failures of food-producing species [147] (see Irruptions and extralimital wandering). Altitudinal migration between seasons and year-round residency at subalpine elevations are 2 scenarios reported in the literature.

Altitudinal migration: Clark's nutcrackers typically move up and down mountainous topography in response to seasonal food availability. Some studies have reported Clark’s nutcrackers congregating at low elevations during the autumn seed harvesting season [137,167], presumably moving from subalpine habitats because of: 1) the depletion (or nonproduction) of the year’s crop of preferred large-seeded, wingless pine species and/or 2) inclement weather.

In the eastern Sierra Nevada, Clark's nutcrackers migrate annually. Analyzing 4 years of observational data, Tomback [160] concluded most Clark's nutcrackers within her study area moved seasonally between discrete elevation zones. Clark’s nutcrackers utilized subalpine sites during the summer and much of the fall. During this time they recovered whitebark pine seeds cached the previous autumn and spent most of their time harvesting and caching the current seed crop. Clark’s nutcrackers vacated high-elevation whitebark pine habitats in late October, when the subalpine seed resource dwindled. Once on lower-elevation Jeffrey pine sites, Clark's nutcrackers again cached seeds from the current year’s seed crop–Jeffrey pine and singleleaf pinyon–and remained within low-elevation communities throughout the winter and spring. Although Clark’s nutcrackers foraged opportunistically for seeds remaining in Jeffrey pine cones, they relied primarily on low-elevation seed caches for food during overwintering and breeding periods. Clark’s nutcrackers and their young returned to high-elevation whitebark pine communities in late spring, when subalpine weather conditions become more moderate. Although the majority of birds followed this cycle, some individuals and pairs remained on subalpine sites throughout the year [160].

In a review, Tomback [147] indicates that in addition to those in the eastern Sierra Nevada, many Clark’s nutcracker populations apparently undergo a seasonal altitudinal migration. She cites the following observational studies showing apparent adherence to this pattern: Giuntioli and Mewaldt [38] in the Northern Rocky Mountains of Montana, Tomback and Kramer [155] in the Sierra Nevada of California, Torick [161] in the Colorado Front Range, and Vander Wall and Hutchins [172] in the Southern Rocky Mountains.

Year-round residency: The literature also reports year-round residency within high-elevation habitats. In Wyoming, observational studies indicate Clark’s nutcrackers stayed at subalpine elevations throughout the fall, harvesting and caching whitebark pine seeds. Individuals cached until the moderate seed crop was depleted; by 4 November Clark's nutcrackers were totally dependent on seed caches for food [41]. In the Washington Cascade Range, Lorenz [85] tracked movements of radio-tagged Clark's nutcrackers over portions of 2 years. She concluded resident Clark’s nutcrackers within this population did not undergo altitudinal migration. Instead, they maintained a stable high-elevation, year-round home range that encompassed the majority of seed caches made during the autumn harvest season. Although Clark’s nutcrackers congregated at low elevations in the fall to harvest ponderosa pine and Douglas-fir seed, resident individuals did not cache in low-elevation harvest stands. Instead, they almost always transported seeds back to high-elevation portions of the summer range for caching [85].

Irruptions and extralimital wandering: Simultaneous seed crop failures of major seed sources precipitate periodic irruptions in Clark’s nutcracker populations. Large numbers of Clark's nutcrackers emigrate from their home regions, making long flights in search of alternative foods [28,29,36]. Irruptions typically start in late summer, when Clark's nutcrackers begin foraging on and caching the new seed crop [160]; absence of green cones may be a triggering factor [171]. Emigrating Clark’s nutcrackers may be adults, juveniles, or both [28]. Juvenile birds may be more prone to emigrate than adults because they are less efficient at seed caching and may have inadequate food stores to overwinter successfully [30]. Emigrating individuals are often underweight and in "poor condition" [28,29,169]. Mortality may be high for Clark's nutcrackers overwintering outside the normal breeding range; unfamiliar terrain, foods, and predators probably increase mortality over that of birds wintering within the breeding range [171].

Major irruptions outside the normal breeding range (extralimital wandering) occur irregularly, often at intervals of 5 to 15 years [28,29,36]. Davis and Williams [28,29] report Clark’s nutcrackers irrupted at least 6 times from the Sierra Nevada. Between 1889 and 1964, birds wintered in low-elevation desert or coastal areas, often 120 to 190 miles (200-300 km) from the normal breeding range. During the winter of 1955 to 1956, Clark’s nutcrackers erupted from the Sierra Nevada to the Monterey Peninsula, where they ate suet from bird feeders, insects from overturned cow dung, and yellow jackets (Vespinae); irruptive birds also foraged on the seeds of Deodar cedar (Cedrus deodara) and perhaps Canary Island pine (P. canariensis) and Monterey pine (P. radiata) [28]. In Canada, Clark’s nutcrackers wandered 1,100 miles (1,800 km), from western Alberta to central Ontario, to find suitable food sources [36]. Extralimital occurrences of Clark's nutcrackers are recorded in Pennsylvania, Illinois, and Arkansas [147].

Minor irruptions within normal breeding range may occur frequently, redistributing Clark's nutcrackers in response to local food shortages. In 1977, Vander Wall and others [171] reported that Clark’s nutcrackers emigrated south through northern Utah from late August though early October in flocks of 11 to 42 birds and probably overwintered in pinyon-juniper woodlands to the south. A northward movement of Clark’s nutcrackers the following summer suggested the subsequent return of some individuals. Most emigrating Clark's nutcrackers were adults and did not cache seeds. The authors suggest that Clark’s nutcrackers wander extensively within their normal breeding range and habitats, searching for suitable food sources before exploring extralimital areas. Birds irrupting within the normal range probably recuperate quickly and experience relatively low mortality [171].

Information from radio-tagged Clark's nutcrackers [85] suggests 2 components of the Clark's nutcracker population—residents and emigrants—occur within the same region.

• Plant species composition
• Succession
• Habitat characteristics
• Habitat use
• Landscape use

Nutcracker pines: Seed caching by the Clark’s nutcracker is especially important in seed dispersal for all 5 of these pine species. Because these pines produce large, heavy, wingless seeds that are retained to some extent within the cones, they depend upon bird-mediated seed dispersal and are collectively called “nutcracker pines” [156]. When seeds of these species are available, Clark’s nutcrackers generally prefer to consume and cache their large, wingless seeds. Nutcracker pines include whitebark pine [41,137,141], limber pine [15,80,155,167], southwestern white pine [121], singleleaf pinyon [79,137,167,169], and Colorado pinyon [169]. Although undocumented, the Clark’s nutcracker may also occasionally disperse the seeds of Parry pinyon (P. quadrifolia) and Mexican pinyon (P. cembroides)where they occur within its range [145].

The Clark’s nutcracker is considered a seed dispersal mutualist of these 5 large-seeded pine species. Pines benefit from the scatter-hoarding behavior of the Clark’s nutcracker, ultimately gaining both consistent seed dispersal to sites favorable for seedling establishment and seed concealment from predators; the Clark’s nutcracker gains an energy-rich, efficiently harvested food source that can be stored for later use [141].

Of the nutcracker pines, whitebark pine is considered an obligate mutualist: essentially all ecologically meaningful whitebark pine seedling establishment results from unclaimed Clark's nutcracker caches [42,75,140,141,156]. The Clark’s nutcracker and whitebark pine are thought to represent a coevolved mutualism [72,141,148,156]. Whitebark pine is the only stone pine species (Cembrae) native to North America; Europe has four species of Cembrae pines and at least 10 subspecies of European nutcracker (Nucifraga caryocatactes). Morphological similarities between whitebark pine and limber pine apparently result from adaptive selection. Historical origin of Clark's nutcracker-dispersed pines is further discussed by Lanner [71], Tomback [141,148,149] and Tomback and Linhart [156].

Winged-seeded conifers: Besides utilizing the large, wingless seeds of the nutcracker pines, Clark’s nutcrackers also frequently consume and cache winged-seeded species that grow close to stands of preferred species. Both large- and small-seeded species may be utilized and include Jeffrey pine [137], ponderosa pine [15,85], Rocky Mountain bristlecone pine [15], Great Basin bristlecone pine (unknown if cached) [74,79], and Douglas-fir [75,85]. Seeds of foxtail pine may also be utilized [125].

Organization of the next sections of this review is based on seed attributes of the conifers listed below. For information on the general ecology and fire ecology of these species, follow the links in the table to the FEIS reviews of these conifers.

Succession: Most of the pines cached by Clark’s nutcrackers are pioneering species that regenerate during early succession. The caching behavior of the Clark’s nutcracker helps maintain these species in disturbed areas [159]. Germination of unretrieved seed in Clark's nutcracker caches may result in the following: pine colonization of previously unforested sites; seedling establishment beneath open to semiopen stands in gaps in the forest canopy; and/or widespread pine regeneration in newly created openings following fire or other disturbance. Generally speaking, Clark's nutcracker caching may eventually produce persistent, self-sustaining stands of preferred pine species on xeric sites where disturbance is infrequent; caching on mesic sites typically results in broadly even-aged stands of preferred pine species that dominate early successional stages but are eventually replaced by more shade-tolerant species in the absence of disturbance [6,8,112,124,173].

Climax (xeric) stands: On harsh, exposed sites, the caching habits of Clark’s nutcrackers tend to produce pure, persistent, open stands of large-seeded, wingless pines; on these sites, shade-tolerant competitors grow poorly and disturbance is infrequent [41,124]. Unretrieved caches on open, rocky, exposed or communal storage sites often result in initial seedling establishment under favorable conditions. Over time, continued caching and germination of retrieved seeds produces new pine regeneration, and may eventually result in an all-aged stand structure. A population of Clark’s nutcrackers may continue to cache in the same stand for decades or centuries. Ages of limber pines in a Utah grove ranged from 24 to 1,000 years [77].

Occasional seedling establishment from cached seed is responsible for the self-sustaining nature of climax stands. The species favored on these sites are long lived, and the stands are able to persist indefinitely in the absence of disturbance. Although reaching reproductive maturity may take many decades, individual trees can function as a seed source for centuries or a millennium [124].

Seral (mesic) stands: Clark’s nutcrackers may cache heavily in forest openings created by disturbance, including fire [141]. Although germination of unretrieved seed may result in abundant regeneration of large-seeded pines for several decades following disturbance, pines are usually seral on productive sites, and successional replacement occurs in the absence of disturbance. Large-seeded pines sometimes act as nurse trees on mesic sites, moderating the harsh early seral environment and facilitating seedling establishment by shade-tolerant associates such as Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa) [114,159]. Establishment of shade-tolerant associates eventually creates mixed-species stands.

It is unclear whether mixed stands continue to be attractive caching habitat, especially during midsuccessional stages. Several studies document a decrease in seedling establishment of limber pine [33,114,127] and whitebark pine [21] in midseral stands. Seedling recruitment of pioneering species ends sooner on mesic than on xeric sites [33]. Apparently limber pine seedling establishment is extremely rare in late-successional stands [133]. Although established seedlings are one measure of caches that were not recovered, they represent only a portion of the caches made in an area. Clark’s nutcrackers might still be utilizing seral stands as cache sites, but seedling recruitment by shade-intolerant pines likely declines under light-limited conditions (see Caching habitat).

Habitat characteristics: The caching patterns and preferences of Clark’s nutcrackers are in large part responsible for the elevational and geographical occurrence of preferred pine species. Nutcracker pines often have a patchy distribution on the landscape [71,156].

A signature of seed dispersal by animals is the occurrence of regeneration clusters—even-aged groups of seedlings, saplings, or mature trees originating from separate seeds within a single cache. The caching behavior of the Clark’s nutcracker often produces clusters of mature trees in whitebark and limber pine; clusters may have fused or contiguous trunks but actually consist of 2 or more genetically distinct individuals (different genets) [22,81,156]. Colorado pinyon typically exhibits a single-trunked growth form at maturity even when originating from Clark's nutcracker or pinyon jay (Gymnorhinus cyanocephalus) seed caches [158]. Studies of tree clumps along the Colorado Front Range indicate multigenet tree clusters routinely occur in ponderosa pine and Rocky Mountain bristlecone pine, 2 wind-dispersed pines often cached by Clark’s nutcrackers [162]. Other wind-dispersed species producing a multistemmed growth form that may be diagnostic of Clark’s nutcracker caching include Great Basin bristlecone pine and Rocky Mountain Douglas-fir [74]. Caching by the Clark’s nutcracker is not responsible for a common, morphologically similar growth form where multiple tree trunks are the result of a single tree branching at the base (forked tree). See Tomback and Schuster [158] for a review of growth form distributions in nutcracker-dispersed pines.

Although Clark's nutcrackers play an important role as the initial dispersal agent for many wingless, large-seeded pines, the extent to which Clark's nutcracker dispersal results in seedling establishment is unclear. Reviews of pine seed-fate studies suggest that bird-mediated dispersal of large-seeded pines may not be a "single-phase" event. Reviews indicate scatter-hoarding rodents are often effective secondary dispersal agents for many pine species. Scatter-hoarders pilfer pine seeds cached by birds, squirrels (Scuridae), and other rodents; once pilfered, seeds may be recached and pilfered multiple times, frequently by different individuals. Scatter-hoarding rodents that harvest whitebark pine seeds include chipmunks (Tamias spp.), ground squirrels (Ammospermophilus and Spermophilus spp.), and deer mice (Peromyscus maniculatus) [10,86]; their effectiveness in promoting successful whitebark pine regeneration was undocumented in the literature as of 2008.

Habitat use:
Foraging habitat: Clark’s nutcrackers forage most efficiently in open to semiopen stands of preferred pine species. Open-grown trees have large crowns and tend to produce the largest seed crops [91]. Clark’s nutcrackers generally select trees (and stands) with higher cone densities and cones with higher proportions of edible seed [130,137,155,167,169]. Trees growing in dense, mixed-species (seral) stands generate smaller seed crops because small, narrow crowns produce cones only near the top of the tree [41]. Although Clark’s nutcrackers may forage in mixed stands, foraging efficiency tends to be sacrificed [41,94] (see Arrangement of seed caches).

Pine squirrel competition for cones: Open stands are also less likely to be heavily used by pine squirrels (Tamiasciurus spp.): Douglas's squirrel (T. douglasii) in the Cascade Range and Sierra Nevada and red squirrel (T. hudsonicus) in the Rocky Mountains. Because pine squirrels cut and cache whole cones of many conifer species, they are a major preemptive seed competitor throughout much of the Clark’s nutcracker’s range [16,130]. Red squirrels are not attracted to dry, relatively open sites where large-seeded pines occur because alternative foods such as fungi are lacking, and predation upon squirrels may be high [16]. Reinhart and Mattson [115] reported low densities of resident red squirrels in open, nearly pure whitebark pine stands in Yellowstone National Park, Wyoming. Red squirrels apparently preferred to establish territories in mixed-conifer stands, where access to cones from a variety of species might off-set years of poor whitebark pine cone production. Clark’s nutcrackers harvested 99.4% of the whitebark pine seed from open-grown trees located on moraine ridges within a subalpine meadow. These small tree islands were at least 700 feet (200 m) from the nearest forest edge and had a stand structure too open for red squirrels to utilize [41]. By comparison, Clark’s nutcrackers harvested only 36.1% of whitebark pine seeds in nearby, contiguous subalpine fir-Engelmann spruce stands, while red squirrels harvested 63.7% of the seeds [41].

In mixed, seral stands, pine squirrels occur in high numbers and rapidly remove the cones of large-seeded pines before Clark’s nutcrackers begin harvesting individual seeds. Red squirrels prefer whitebark pine seed over seed of other conifer species, often removing whitebark pine cones before Clark’s nutcrackers can harvest the seeds [41,94]. In a study on subalpine sites in Wyoming, Hutchins and Lanner [41] reported more rapid depletion of seeds borne on whitebark pines in forested sites than on trees in open sites. Seed harvest began earlier in forested sites, with 50% of the whitebark pine seed crop harvested by the start of Clark’s nutcracker caching (end of August); in open meadow stands where only Clark’s nutcrackers harvested seed, the 50% depletion point occurred nearly a month later. Red squirrels also reduced Clark’s nutcracker access to southwestern white pine cones in mixed-conifer forests of the San Juan Mountains. On these sites, southwestern white pine exhibits a very protracted cone-opening phenology, and red squirrels continued to cut southwestern white pine cones as long as closed cones were available. Samano and Tomback [121] found that red squirrels were less numerous and removed fewer cones in open, mixed-conifer forests than in closed-canopy forest. Only 7% of the southwestern white pine cones remained on monitored trees by 30 August [121].

Clark’s nutcrackers tend to be more abundant where pine squirrels were historically absent. Comparing Clark’s nutcracker abundance in mountain ranges with pine squirrels (Sierra Nevada and Rocky Mountains) to those without pine squirrels (most of the Great Basin and the Sweet Grass Hills of Montana) over a 2-year period, Siepielski and Benkman [130] found Clark’s nutcrackers were more than twice as abundant in mountain ranges not inhabited by pine squirrels; the difference was significant whether the site was dominated by whitebark pine (year 1: P≤0.013; year 2: P≤0.0001) or limber pine (year 1, P≤0.015; year 2, P≤0.013). Tamiasciurus species are uncommon in pinyon-juniper woodlands [16].

Patchy occurrence within the landscape matrix: Stands of preferred species often occur in isolated forest patches throughout Clark’s nutcracker's range. In areas where Clark’s nutcrackers inhabit high-elevation subalpine sites, patches of whitebark pine typically occur as stringers along mountain ridges, form narrow bands of ribbon forest, or are scattered throughout subalpine meadows. Studies investigating bird use of forest patches at the subalpine forest-alpine ecotone in the Beartooth Mountains of Wyoming found Clark’s nutcrackers were more frequently recorded in intermediate-sized forest patches than in large areas of contiguous forest or small, forested fragments [110]. Forest patches used by Clark's nutcrackers ranged from 2.2 to 5.7 acres (0.9-2.3 ha) and were surrounded by open meadow. Two types of forested areas occurred within a matrix of meadow: some were on rocky, well-drained soil; others were narrow stands of ribbon forest about 70 to160 feet (20-50 m) wide. All contained an overstory of whitebark pine, Engelmann spruce, and subalpine fir with a grouse whortleberry (Vaccinium scoparium) understory; the tree canopy generally reached heights of 30 feet (8 m).

Caching habitat: Clark’s nutcrackers scatter-hoard pine seeds in a myriad of sites, aspects, elevations, and forest types. Besides caching in forests dominated by nutcracker pines, Clark's nutcrackers also cache in sagebrush meadows and curlleaf mountain-mahogany (Cercocarpus ledifolius) woodlands [170]. Cache site diversity apparently increases the likelihood of a constant food supply throughout the year. During late fall and winter, some caches are accessible when others are buried by snow; progressive snowmelt creates newly accessible caches at other times of year, especially during spring and summer [71,137,169] (see Cache microsites).

Arrangement of seed caches: Clark’s nutcrackers generally distribute seed caches in 3 types of storage sites within their habitat:

• local storage sites, which are in terrain immediately adjacent to harvest trees
• communal storage sites, which are typically open areas distant from the harvest trees, where the local population of Clark's nutcrackers concentrates food stores [41,160,169]
• overlap between local and communal storage sites. Over time, as succession takes place and open communal sites transition into forested communities, forest stands may function as both local and communal cache sites.

When seeds are plentiful, caching close to the harvest trees maximizes the number of seeds stored. Apparently, some local caches are only temporary [41,137]. As the harvest season starts to wind down, Clark’s nutcrackers may recache locally cached seed, moving it to communal storage slopes or to more open sites where seed predators are less numerous. The extent to which recaching occurs within or between Clark's nutcrackers populations is unclear. Recaching may be more prominent in some areas than in others [41,137] (see Recaching).

Communal storage sites: Clark’s nutcrackers use open areas within mountainous landscapes for communal caching. Since strong winds and harsh exposures keep the majority of communal cache sites relatively free of snow, large numbers of Clark’s nutcrackers utilize these areas [85,160]. Clark’s nutcrackers often transport seeds a considerable distance in order to cache within communal storage areas, perhaps indicating a preference for forest openings. A Clark’s nutcracker population may use one or more communal caching sites within a locality. Forest openings used as communal storage sites include steep, south-facing open slopes and openings in the forest canopy [41,85,121,137,169].

Steep, south-facing communal slopes: Clark’s nutcrackers appear to cache preferentially on steep, southerly, windswept open slopes and ridges within their home range: areas characterized by little snow accumulation and rapid snow melt during winter and spring [41,85,160,169]. These open areas may remain snow free even after severe snow storms, optimizing Clark's nutcracker access to seed caches especially during winter and early spring. Clark’s nutcrackers commonly select the most open and exposed portions of these slopes for communal caching; slopes are dry and typically have little vegetation [30,160].
		South-facing slopes and cliffs used communally in the upper subalpine zone of the Cascade Range. Photo by Nicholas T. Ernst.

Communal slopes that are steep and south-facing tend to be used throughout the range of the Clark’s nutcracker. For instance, in the central Sierra Nevada, the 3 communal storage areas near Mammoth Mountain were large, bare, steep (inclinations of 22-30°), south-facing slopes covered by a deep layer of pumice or gravelly soil [137]. In the San Francisco Mountains in Arizona, Clark’s nutcrackers used an opening within a mixed conifer community as a communal caching ground; it encompassed a south-facing, 35° slope and was approximately 6.2 acres (2.5 ha) in area in area [169]. See reviews by Tomback and others [145] and/or Lorenz and others [86] for further details of communal slope site characteristics.

Forest openings used communally: Clark’s nutcrackers also cache seeds in other open habitats such as rock strewn ridges, rocky outcroppings, cliff faces, open meadows, meadow swales, alpine areas, and newly created forest openings such as burns and clearcuts [43,62,143,153]. Clark’s nutcrackers heavily utilize recently created openings for caching and often fly long distances to use them [137,148]. (But see Lorenz and Sullivan 2008, cited in [10].)

Criteria used by Clark’s nutcrackers to select communal caching grounds have not been quantitatively evaluated. Minimal vegetation appears to be a common characteristic [139], perhaps because seed cache predation by rodents is less likely in areas of sparse vegetation. Topographic features that maintain forest openings with minimal vegetation and maximize the snow-free period apparently provide communal cache sites. Likewise, newly created forest openings also offer sparse vegetation and a break in the forest canopy, although for comparatively brief periods and with varying amounts of snow accumulation.

Studies in the Washington Cascade Range found differences in caching preferences between resident (nonmigratory) and emigrant Clark's nutcrackers. Researchers observed radio-tagged, resident individuals caching whitebark pine during the autumn harvest season. Residents placed 99% of caches outside the communal caching grounds, caching instead within their year-round home range. Residents also placed 97% of these caches in locations where successful seedling establishment was unlikely—in cliffs, tree canopies, and midelevation ponderosa pine-Douglas-fir communities. Emigrants, however, placed 100% of whitebark pine caches in high-elevation whitebark pine stands that researchers considered communal caching grounds (Lorenz and Sullivan 2008, cited in [10]).

Overlap between local and communal storage: Although most communal storage slopes are not heavily forested, some slopes may eventually support all-aged stands as pine trees originating from unretrieved caches become more numerous [139,148]. When trees within all-aged stands start to produce cones, Clark’s nutcrackers begin caching beneath the canopy, essentially caching locally on what was previously an open communal slope. This type of caching occurs frequently on late-seral sites and has been observed in the Sierra Nevada [137], in the Northern Rockies (Wind River Range, Wyoming) [41], and the Washington Cascade Range [87].

Seed cache predation: Although Clark’s nutcrackers bury seed and scatter seed caches to minimize the likelihood of predation, high rates of seed predation nevertheless occur. Seed caches located in open terrain are generally subjected to less predation than those in more forested sites. On subalpine sites in Wyoming, for example, Hutchins and Lanner [41] report higher survival of whitebark pine seed cached artificially in an open meadow than in a mixed stand containing whitebark pine; caches were made in the fall at 0.4-inch (3 cm) depths to simulate Clark’s nutcracker caching. Whereas 100% of seeds cached in the meadow survived to the following summer, only 43.3% of seeds cached in the forest survived. Similar trends occurred the following year, with 62.9% cache survival in the meadow (n=8) and 10.0% survival in the forest (n=12) [41]. High rates of seed predation are reported elsewhere in the literature [92,137].

Extent of seed cache predation also varies with vegetation zone. Studies of artificial caches of limber pine seed from the Colorado Front Range indicate rodents routinely deplete Clark’s nutcracker caches. Mean predation tended to decrease as cache site elevation increased, as did densities of deer mice and chipmunks. Predation was lowest on alpine sites (13%) and highest (60%) on montane sites [15].

Variation in caching patterns: Patterns of food storage in Clark’s nutcrackers appear complex and reflect a specific population’s use of the local habitat [10,86,137]. Clark’s nutcracker populations exhibit flexibility in the use of local and communal storage sites, and utilization patterns vary considerably within and among geographical regions. A population may use one or more communal caching sites, the caching sites may be at higher or lower elevations relative to the harvest sites, and caching flights may involve distances of up to 20 miles (32 km). Much of the literature reflects observational studies conducted over the range of the Clark’s nutcracker. Recent information from radio-tagged individuals in the Washington Cascade Range indicates Clark’s nutcracker habitat use may differ according to the migratory status of individuals [85].

Cascade Range: In the Cascade Range of Washington, Lorenz [85,88] used radio-tagged Clark’s nutcrackers to study caching behavior of a Clark's nutcracker population. Although resident Clark's nutcrackers routinely made long-distance forays to locate seed, they always returned to their year-round home range for caching. Trips transporting seeds between harvest stands and cache sites sometimes covered a distance of up to 18 miles (29 km) and changes in elevation of 3,304 feet (1,007 m). In contrast, when emigrant Clark's nutcrackers harvested seed they cached either locally near harvest trees or on nearby communal slopes; generally cache sites were less than 3 miles (5 km) from the harvest site. Once spring arrived, communal slopes functioned as communal cache-retrieval sites and provided a concentrated food source for the emigrant segment of the population (see Retrieving caches [85].

An elevational shift in habitat utilization occurred in both the resident and emigrant populations in both years of the study. Residents used elevations between from 5,381 and 6,201 feet (1,640-1,890 m) during the summer. Early in the fall harvest season, residents continued to use high elevation portions of their home range; seed harvest sites had mean elevations of 5,591 feet (1,814 m). Resident use shifted to lower elevations during September in both years of the study (14 September 2006; 21 September 2007); mean elevation of seed harvest observations was 3,365 feet (1025 m). Emigrants also moved into lower-elevation habitats during autumn, feeding on the large ponderosa pine seed crop [85].

Sierra Nevada: A 4-year study by Tomback [137,160] on the east side of the Sierra Nevada found a complex pattern of food storage for Clark’s nutcracker populations in and around Yosemite National Park. Most Clark's nutcrackers underwent a seasonal altitudinal migration, with local and communal caching occurring at both high and low elevations. During the whitebark pine harvest season, Clark’s nutcracker populations used several communal storage sites within the subalpine zone at elevations from 8,900 to 9,800 feet (2,700 - 3,000 m) and also transported whitebark pine seeds to low-elevation communal caching grounds. Once the whitebark pine harvest was complete, Clark’s nutcrackers migrated to low-elevation Jeffrey pine forests at 6,900 to 7,900 feet (2,100 - 2,400 m) elevation, where they harvested and cached singleleaf pinyon and Jeffrey pine seeds. Clark's nutcrackers remained in the Jeffrey pine zone for overwintering and breeding.

Examples of transport and storage patterns from Clark's nutcrackers in the eastern Sierra Nevada are presented below.

Mammoth Mountain area: The Clark’s nutcracker population in the vicinity of Mammoth Mountain utilized at least 3 communal storage areas in addition to local caching. When harvesting whitebark pine from the subalpine, Clark's nutcrackers would either fly 1.6 miles (2.5 km) down the west slope of Mammoth Mountain to a communal storage area about 660 feet (200 m) lower in elevation or head 7.8 miles (12.5 km) downslope to a communal storage area at Casa Diablo (Jeffrey pine zone), about 1,640 feet (500 m) lower in elevation in Little Antelope Valley [137].

Tioga Pass: Clark’s nutcrackers harvested whitebark pine seed from trees growing on rocky hillocks scattered throughout a subalpine meadow. They primarily cached seeds locally under the harvest trees; however, they sometimes transported seeds to a lower-elevation communal caching ground on the slope of the steep, south-facing Lee Vining Canyon. At lower elevations, individuals cached among sagebrush and singleleaf pinyon [137].

Northern Rocky Mountains: In the Absaroka Mountains of Wyoming, a study by Hutchins and Lanner [41] reports a simple pattern of food storage. Clark’s nutcrackers inhabiting subalpine sites had only one communal storage area. While waiting for the new seed crop to ripen, Clark’s nutcrackers relied heavily on the previous year's caches; in other areas, Clark's nutcrackers start utilizing unripe seed during this time period. Individuals either cached whitebark pine seeds locally near the parent tree or flew 2.3 miles (3.5 km) to the ledges of the Breccia Cliffs, a steep, south-facing communal storage area. Much of the locally cached seed was later moved to the communal slope, and the population was totally dependent on food stores by early November [41].

Middle and Southern Rocky Mountains: In the Colorado Front Range near Rocky Mountain National Park, Clark’s nutcrackers carried Rocky Mountain bristlecone pine seeds up from a subalpine forest to alpine tundra cache sites [15].

In the San Juan Mountains, Clark’s nutcrackers cached southwestern white pine seeds on a west-facing communal storage slope approximately 0.16 to 0.31 miles (0.25-0.5 km) from harvest stands. Some local caching also occurred in harvest stands where researchers observed recently established seedlings [121]. Clark's nutcrackers were most active mid morning and late afternoon; most foraging occurred on immature, closed cones (cones of southwestern white pine open at maturity) [121].

Southwest: In the San Francisco Peaks of northern Arizona, Clark’s nutcrackers transported Colorado pinyon seeds 4.7 to 14 miles (7.5-22 km) from harvest trees to communal caching grounds; seeds were frequently transported to cache sites 1,740 to 1,970 feet (530-600 m) higher in elevation [169].

Breeding habitat: The Clark’s nutcracker usually uses open, montane coniferous communities for breeding. Montane communities used for nesting are typically low in elevation and include the following: pinyon-juniper woodland composed of Colorado pinyon, singleleaf pinyon, and/or western juniper (Juniperus occidentalis) [137,160]; ponderosa pine [17,98]; Douglas-fir [98]; Jeffrey pine [137]; and grand fir (Abies grandis) [68]. Some Clark's nutcrackers nest in mixed-conifer subalpine communities where either whitebark pine or limber pine are locally abundant [13,136]. In a review, Tomback [147] suggests subalpine nesting is underreported.

Nest sites are usually located near seed stores made the previous fall [147]. Clark’s nutcrackers appear to select sheltered sites that offer some protection against the prevailing winds [160]. Clark's nutcrackers may also prefer small stands of trees located near the forest edge or above valley bottoms [17,31,137]. Nest trees are often on south-facing slopes [98] or in canyon bottoms [17,19,137], where snow melt is rapid and seed stores are easily accessed.

Nest tree species vary with forest community type and geographical region.

Clark’s nutcrackers construct their nests in a variety of locations within a tree canopy. Within pinyon-juniper communities, where nest trees are short in stature, nests are 7 to 20 feet (2 to 5 m) above the ground; in communities where potential nest trees attain greater height, nest placement ranges from 5.9 to 79 feet (1.8-24 m) above the ground [147]. Mewaldt [98] found a diversity of nest placements within a ponderosa pine/Douglas-fir forest in western Montana; nest placement included flush up against the tree bole, 4 feet (1 m) out from the trunk on a lateral branch, and towards the end of a heavily foliaged branch.

Roosting habitat: Little published information existed on the roosting habitat of Clark’s nutcrackers as of 2008. Tomback [138] suggests Clark’s nutcrackers may extend foraging opportunities by selecting roost sites that receive early morning light. She reports flocks of 30 to 70 individuals roosting in a dense stand composed of whitebark pine, lodgepole pine, and red fir (Abies magnifica) at 10,330 feet (3,150 m) elevation in the Sierra Nevada. The stand was located on a steep, east-facing slope of a major ridge line and received first light at sunrise. Clark’s nutcrackers congregated on a steep west-facing slope just prior to sunset, perhaps attempting to extend foraging time by remaining on the last sunlit stand in the vicinity; as daylight waned, Clark's nutcrackers flew to the east-facing roost sites [138].

Landscape use: Much of the information regarding space utilization by the Clark's nutcracker has been largely speculative. Studies using radio-tagged individuals indicate that a local population of Clark's nutcrackers is made up of 2 groups that use the landscape differently.

Space utilization patterns: For portions of 2 years, Lorenz [85] followed the movement of radio-tagged Clark’s nutcrackers in the Cascade Range of Washington. Comparing space utilization between summer and autumn, Lorenz concluded that Clark’s nutcracker space utilization varied according to an individual’s migratory status. She grouped Clark's nutcrackers into 2 categories: residents and emigrants. Lorenz and Sullivan (2008, cited in [10]) suggested that individuals may readily switch status between years depending on cone production. The model of differential space use proposed by Lorenz [85] builds upon a model suggested by Vander Wall and others [171], who coined the term “emigrants” for the nonresident segment of the population.

Residents: Lorenz considered 5 of 26 radio-tagged individual resident Clark's nutcrackers. Each maintained a stable summer home range, where birds foraged for daily energy requirements and remained until the start of seed harvest. During autumn, the home range expanded as a result of long-distance forays to obtain seeds for caching. Because Clark’s nutcrackers routinely transported seeds back to the summer home range for caching, summer home ranges were contained within fall home ranges. From preliminary results, Lorenz speculated the summer home range represents the year-round home range of residents (except for autumn forays) because: 1) individuals cached almost exclusively within the summer home range; 2) seed caches are food stores used for winter survival and spring breeding; and 3) day and night roosting, preening and other social interactions consistently occurred within summer range [85].

During the harvest season, resident Clark's nutcrackers first harvested and cached seeds produced by whitebark pine stands within their home range. As home range seed resources became depleted, Clark's nutcrackers began ranging greater distances outside their home range, traveling up to 19 miles (30 km) from their home range in search of whitebark pine stands [85].

Emigrants: Although Lorenz [85] collected less data from the emigrant segment of the population, emigrant Clark's nutcrackers exhibited a pattern of habitat utilization marked by wide ranging movements and opportunistic use of seed resources. During spring and early summer, emigrants commonly settled into 1,000- to 2,000-acre (400-1,000 ha) patches of the landscape for 2 to 7 days at a time; they would then relocate to a new landscape patch approximately 6 to 25 miles (10-40 km) away. For instance, during May and June, one emigrant female used 4 distinct patches within in a 50-km² area; she then moved 106 miles (170 km) north of the first area, where she ranged over 274 km² during the month of August [85].

Emigrant foraging was characterized by harvesting and caching on a landscape scale (Lorenz and Sullivan 2008, cited in [10]). During the whitebark harvest season, emigrants were highly mobile, often ranging over more than 260 km²; most caching occurred in communal caching grounds near the harvest trees (Lorenz and Sullivan 2008, cited in [10]).

Home range: Clark’s nutcrackers are nonterritorial outside the breeding season. In a review, Tomback [147] reports their movements are confined to a home range that probably corresponds to the location of seed stores and reflects seasonal patterns of elevational migration and seed use.

Cascade Range: Lorenz [85] followed the seasonal movements of 5 radio-tagged resident Clark's nutcrackers in the Washington Cascade Range. Mean total home range size of resident Clark's nutcrackers (n=4) was 8,189 acres (3,314 ha). Summer home range estimates of residents was 786 acres (318 ha); autumn home range estimates of residents was 2,170 acres (880 ha). Autumn home range size was larger than summer home range size because Clark's nutcrackers made long distance seed harvesting forays [85].

Resident Clark's nutcrackers cached only in the higher elevation portions of their summer range; although they harvested seed at lower elevations, Clark's nutcrackers continued to cache within their summer home range throughout the autumn. Individual core areas overlapped between 2 Clark's nutcrackers occupying home ranges within a ponderosa pine forest; overlapping core areas also occurred in 3 residents occupying whitebark pine home ranges [85].

Year-round home range characteristics varied among the 5 radio-tagged residents. Tree species composition of all 5 home ranges included both ponderosa pine and Douglas-fir. Three home ranges occurred along a ridgeline; whitebark pine and mountain hemlock (Tsuga mertensiana) comprised the overstory on the north-facing slope of the ridge with ponderosa pine, Douglas-fir, and grand fir on the south-facing side. The other 2 home ranges occurred within an open, southwest-facing forest where a mixed-severity fire had burned through the area 4 years earlier; ponderosa pine and Douglas-fir comprised the overstory on these sites [85].

Home range size may be influenced by tree species composition. Clark's nutcrackers with home ranges containing mature whitebark pine had an average home range of 2,550 acres (1,030 ha); Clark's nutcracker home ranges without whitebark pine averaged 7,680 acres (3,110 ha) [83].

Sierra Nevada: Observational studies in the eastern Sierra Nevada indicate Clark's nutcrackers cached seeds at both subalpine and lower montane elevations and exhibited discrete summer and winter home ranges. During late spring and summer, family groups and nonbreeders occupied a home range of approximately 490 acres (200 ha) on subalpine sites [160]. Home range size of Clark's nutcrackers overwintering on lower-elevation Jeffrey pine sites was not given.

Cache microsites: When burying seed, Clark’s nutcrackers select sites with similar microsite characteristics, often near objects. Typical cache microsites include the following: on the forest floor around the base of trees and under tree canopies, beside rocks, among tree roots, next to fallen trees or large branches, and at the base of annual or perennial plants [41,137,153]. Cache substrates include mineral soil, gravel, pumice, rocky rubble, forest litter, burned litter, and dense moss [30,41,137,153]. On moist sites in the Northern Rocky Mountains, seeds are cached in the following sites: wet moss, at the edges of meadows, near springs and streambanks, and sometimes in puddles. Frequently, caching occurs out in the open where no object or landmark is discernible [41].

Although seed is usually buried within the ground, some caches are placed in other kinds of sites, including cliff ledges, rock crevices, fissures in rock faces, and cracks or holes in trees and logs [30,42,88,137,155,160].

The relative occurrence of caches in above- and belowground locations may vary between populations inhabiting different geographical regions. Aubry and others [10] suggest Clark's nutcrackers tend to place more caches in the ground in areas of light regional snowpack; correspondingly, where snow depths are substantial, Clark's nutcrackers may regularly cache in aboveground locations to ensure access throughout the winter. From observations (n=210) of 5 radio-tagged, resident Clark’s nutcrackers in the Washington Cascade Range, Lorenz [88] reports the majority (63%) of cache placements occurred in aboveground locations. Clark's nutcrackers primarily selected trees for aboveground cache sites; seeds were placed within the tree canopy at the ends of branches, under pieces of bark, and within patches of lichens. Due to aboveground caching, forested areas contained the majority of caches; 67% of caches occurred on forested sites (51-100% canopy cover) versus 33% of caches in openings (0-50% canopy cover). Snow depth may also influence caching at the local scale. At high elevations near the crest of the Washington Cascade Range, resident Clark's nutcrackers made 70% of caches within trees; the rest of their caches were in soil. In the same study area, resident individuals occupying dry, low-elevation, ponderosa pine home ranges placed only 36% of caches in aboveground locations, whereas the majority of caches occurred in the ground (Lorenz 2008, unpublished data cited in [10]).

• Diet
• Conifer seed characteristics
• Foraging and caching

Diet: The Clark’s nutcracker specializes in pine seed consumption but is an opportunistic feeder. Like many Corvids, Clark’s nutcrackers are omnivorous, consuming plant material, insects (Hexapoda), spiders (Araneae), small animals, and carrion. Giuntoli and Mewaldt [38] studied the stomach contents of 426 Clark’s nutcrackers from Montana. By volume, stomachs contained, 83% conifer seeds, 13% arthropods (Arthropoda), and 3% carrion. Nearly all stomachs (98%) contained seeds, 59% contained arthropods, and 12% contained animal remains. See Tomback [147] for a monthly breakdown of stomach contents.

Conifer seeds: The Clark’s nutcracker’s year-round diet consists primarily of fresh and stored pine seeds. As discussed earlier (see the table in Preferred habitat), Clark’s nutcrackers prefer the large, highly nutritious seeds of the 5 nutcracker pines [147]. Clark’s nutcrackers use their sturdy bill to crack open the thick seedcoat of these pines, and this is one probable origin of the common name “nutcracker” [50]. Clark’s nutcrackers also feed on the smaller, winged seeds of other pine species and Douglas-fir [15,38,74,79,85,98,137,161,169]. Clark's nutcracker reliance on conifer species is related to the geographical distribution of preferred seed species.

Tomback [145] speculates that 2 large-seeded pines whose northern distributions are within the distributional range of the Clark’s nutcracker may occasionally be utilized: Parry pinyon and Mexican pinyon.

Other foods: The Clark’s nutcracker consumes insects and spiders year-round, gleaning them from vegetation, soil, and fallen cones [160]. Giuntoli [37] found the 3 most frequently occurring insect orders in Clark’s nutcracker stomachs included beetles (Coleoptera), bees and ants (Hymenoptera), and grasshoppers and crickets (Orthoptera).

Clark’s nutcrackers prey on small vertebrates whenever possible. Prey items include eggs, nestlings, and adults of other birds as well as ground squirrels, chipmunks, voles (Muridae) and toads (Bufonidae) [32,160]. Northern pocket gophers (Thomomys talpoides)are also a prey species [27]. Meat may be of particular importance during the nestling period [32,160]. During May and June in a subalpine meadow in California, Clark’s nutcrackers preyed upon mountain voles (Microtus montanus), Belding's ground squirrels (Spermophilus beldingi), a mountain pocket gopher (Thomomys monticola), a Yosemite toad (Bufo canorus), and a white-crowned sparrow (Zonotrichia leucophrys) nestling [105]; approximately half of the predatory attempts were successful. The Clark’s nutcracker also kills and consumes tadpoles of the Columbia spotted frog (Rana luteiventris) in montane wetlands in Idaho [113].

Clark’s nutcrackers consume road kill and other carrion when available. Clark's nutcrackers also visit bird feeders, trash bins, and camping sites for food scraps, particularly during winter storms [160]. In Rocky Mountain National Park, Clark's nutcrackers frequent scenic turnouts, where they consume a variety of food handouts from tourists [143].

Juniper "berries" (cones) are a frequent Clark's nutcracker food in conifer-sagebrush zones during winter, especially during severe storms [160].

Conifer seed characteristics The seeds of preferred pine species have a patchy occurrence in space and time. Irregular cone production, variations in seed crop size and distribution (locally and regionally), differential cone ripening within and among trees of the same species, and the frequently scattered distribution of preferred species within montane habitats create an erratic food resource. Opportunistic, flexible use of food resources enables the Clark's nutcracker to respond to variations in food supply [167].

Attributes of the nutcracker pines: All nutcracker pines have traits that increase Clark's nutcracker foraging efficiency: large, wingless seeds, cones lacking scale spines, and horizontally directed cones on tips of "lyrate" (vertically oriented) branches [71,156]. For example, whitebark pine exhibits a suite of cone presentation characteristics that draws the attention of Clark’s nutcrackers. Cones are conspicuous when viewed from above, visually cueing Clark’s nutcrackers that are searching for food. Display traits include broom-shaped crowns of mature trees that produce extensive cone-bearing surfaces at the tips of branches, where cones are clustered in groups of 2 to 5; short, stiff needles that do not conceal ripening cones; purple cones (instead of green); and cones that, upon ripening, produce beads of oleoresin that glint in the sunlight [72]. These traits are also thought to reduce search time as a Clark’s nutcracker moves from one cone to another within the same tree crown [72].

Most nutcracker pines have varying degrees of seed retention in cones. Whitebark pine has indehiscent cones. Singleleaf pinyon and Colorado pinyon have seeds that are restrained on the cone scales by flanges after the cones open; flanges disintegrate following first frost [13]. Both limber pine and southwestern white pine retain seeds for a few weeks after cones open, requiring prompt harvesting by Clark’s nutcrackers [121,137,156].

Although only pinyon pines (Cembroides) and whitebark pine produce truly wingless seeds, short remnant wings are sometimes found on limber pine and southwestern white pine seeds [72]. On sites in the San Juan Mountains, researchers recorded wings on approximately 12% of southwestern white pine seeds; Clark's nutcrackers apparently avoided these seeds when foraging [121].

Attributes of other winged-seed conifers: Clark’s nutcrackers usually deplete seeds from preferred pines before switching to sympatric, wind-dispersed species [137,161,169]. In the Colorado Front Range, Clark’s nutcrackers consumed limber pine seeds until seeds were depleted, even though ponderosa pine seeds were readily available [161]. Tomback [137] observed an exception to this pattern at low elevations in the eastern Sierra Nevada, where Clark’s nutcrackers preferred harvesting wind-dispersed Jeffrey pine seeds over singleleaf pinyon seeds when both were available; apparently the singleleaf pinyon cones were highly resinous and thus difficult to harvest before completely open. Following radio-tagged Clark’s nutcrackers in the Washington Cascade Range, Lorenz [88] observed one Clark’s nutcracker routinely caching only ponderosa pine and Douglas-fir seeds even though whitebark pine seeds were available and cached by other radio-tagged individuals in the study area.

When utilizing the seeds of wind-dispersed species, Clark’s nutcrackers typically remove the wing before pouching seeds [15,137]. Thus, utilization of winged-seed species involves expenditure of time and energy not incurred while harvesting seeds of wingless species [156]. On sites in the Sierra Nevada, Clark’s nutcrackers dewinged every Jeffrey pine seed before pouching (see Harvesting–feeding and pouching) [160]. Individuals accomplish wing removal by a variety of techniques: shaking the seed to loosen the wing, using the bill to clip off the wing, and rubbing the wing against cone scales, the tree bole, or across needles [13,16,137,156]. Despite wing removal, remaining wing fragments often continue to make pouching difficult [160].

Seed utilization patterns: Over most of its range, the Clark’s nutcracker has access to one or more of the nutcracker pines as a food source in any given year [85,156]. The utilization sequence among preferred species appears linked to cone-ripening and/or cone-opening phenology [145]. In years when more than one preferred pine species is producing cones, Clark's nutcrackers generally harvest and cache seed of the species that ripens first. Whitebark pine seeds are larger than those of most associated pine species except Jeffrey pine [72] and the cones mature early (15 August-25 August) compared to cooccurring pines (September or later), so Clark’s nutcrackers forage preferentially on whitebark pine seed if it is available [148].

On sites in the eastern Sierra Nevada, where Clark's nutcrackers had access to whitebark pine and limber pine seeds and whitebark pine seeds ripened first, Clark's nutcrackers preferred whitebark pine [155]. On sites in the Raft River Mountains of northern Utah, where Clark's nutcrackers had access to limber pine and singleleaf pinyon and limber pine ripened first, Clark’s nutcrackers harvested limber pine before singleleaf pinyon [167]. Clark’s nutcrackers harvested and cached seeds from unripe cones of limber pine and Colorado pinyon concurrently on sites in northern Arizona [169].

Sequential seed utilization by the Clark's nutcracker is reported in a number of studies.

Clark’s nutcrackers prefer harvesting fully ripe seeds that are easily extracted from cones [137,161,169]; as cones ripen, the caloric value of seeds also increases [167]. On subalpine sites in the Sierra Nevada, Tomback [137] reports Clark’s nutcrackers extracted an average of one whitebark pine seed every 31 seconds from unripe cones, an average of one seed every 21 seconds from partially ripe cones, and an average of one seed every 7 seconds from ripe cones.

The cones of nutcracker pines generally ripen and open (except for whitebark pine) asynchronously within and among tree canopies [16,121,137,155,169]. This ripening pattern provides Clark’s nutcrackers a continuous seed source over a period of up to several weeks or more. See Samano and Tomback [121] for a discussion of synchrony patterns in nutcracker pine cone phenology reported in the literature.

Seed crops: Substantial cone production is frequently delayed when conifer species occupy harsh sites. Although trees may reach reproductive maturity within decades, good cone production occurs much later [61,67].

Seed availability of all species used by the Clark's nutcracker varies from year to year and influences their utilization pattern [160]. Good cone production often occurs intermittently with little or no cone production during intervening years; seed crops also vary both locally and regionally [169]. For example, in Lorenz’s study area in the Washington Cascade Range, cone production of the 3 available species varied as follows: whitebark pine—moderate 2006, moderate 2007; ponderosa pine—large 2006, small 2007; Douglas-fir—no cones 2006, large 2007 [85].

Local cone production may also vary from stand to stand within a given year. On sites in Rocky Mountain National Park, Tomback [143] surveyed a limber pine stand where only 10% of trees produced cones, and those trees produced from 1 to 20 cones/tree (x=5). During the same year in another stand, nearly 100% of trees produced cones and those trees produced from 2 to 40 cones/tree. No cones were produced in the 1st stand the following year; on the 2nd site approximately 50% of trees produced cones, and cone production/tree was similar to the previous year [143].

Cone production frequency may vary geographically within a species. For instance, whitebark pine produced moderate to heavy seed crops 4 consecutive years on sites in the eastern Sierra Nevada [137]. In the Greater Yellowstone Area, however, reconstructed whitebark pine cone production from 1980 to 1989 indicates no cone production 48% of the time; in years with some cone production, seed crops were rated as follows: excellent 13%, good 28%, poor 59% of the time [101].

Flexibility in seed species utilization and opportunistic foraging help to buffer the Clark’s nutcracker from the year-to-year variability in harvestable pine seed [156]. However, during years of simultaneous seed crop failures of preferred species, large numbers of Clark’s nutcrackers emigrate outside the breeding range [147] (see Irruptions and extralimital wandering).

Cone competition: Clark’s nutcrackers face fierce competition from pine squirrels when foraging on large-seeded pines in mixed stands. Forest-grown pines usually have small, narrow crowns that produce limited seed crops and form cones only on the very top portion of the crown [41]. Red squirrels are numerous in mixed stands and rapidly remove a considerable proportion of the cones available to Clark’s nutcrackers [16,41]. Researchers on subalpine sites in Wyoming observed that both red squirrels and Clark’s nutcrackers began harvesting whitebark pine seeds at the same time, beginning in mid-July. However, while red squirrels intensively harvested whole cones from mid-July to mid-August, Clark’s nutcrackers frequently tested cones for ripeness but continued to rely heavily on previously stored seed for food until mid-August, when cones matured and caching began [41]. As optimal foragers, Clark’s nutcrackers are unlikely to harvest seeds from trees with only a few cones remaining [130].

		Foraging and caching: Clark’s nutcrackers have a complex sequence of foraging patterns that centers on year-round use of pine seeds as a primary food source. Since pine seeds are plentiful for only a brief time in late summer and early fall, an individual Clark’s nutcracker must harvest and store enough seeds to ensure a food supply for the rest of the year. The seed harvest and storage period lasts 50 to 75 days under favorable conditions and depends on size of the seed crop, the number of Clark's nutcrackers, and competition from other animals foraging for seeds [41,167]. Cache recovery occurs during the winter, spring, and much of the summer [167].
Clark's nutcracker with a full sublingual pouch harvesting whitebark pine seeds. Photo by Nadine Hergenrider.

Morphological and behavioral adaptations: Clark’s nutcrackers are well adapted to harvest seeds from pine cones. They have long, heavy, sharp bills that they use to break open cones. Many of the conifer species utilized by Clark’s nutcrackers have seeds that are retained in the cone after seed ripening. By jabbing and stabbing at cones, Clark’s nutcrackers break off cone scales and use their long, tweezer-like bills to extract seeds [72]. They also use their bills to push seeds into the soil one at a time [137].

To collect and transport seeds, the Clark’s nutcracker uses a sublingual pouch that is unique to the genus Nucifraga. The sublingual pouch is a saclike extension of the floor of the mouth that opens under the tongue [18]. As it removes seeds from cones, the Clark’s nutcracker collects the seeds temporarily in its sublingual pouch, a behavior called "pouching". When its pouch is full, the Clark’s nutcracker transports seeds to cache sites, where the seeds are stored until recovered and eaten. The Clark's nutcracker also uses its sublingual pouch to transport cached seeds to nestlings and dependent juveniles [137].

The flight of Clark’s nutcrackers is characterized as strong, bold, direct, and generally well above the forest canopy [169]; long, pointed wings are adaptations for strong flight. Use of updrafts and winds to gain lift in mountainous landscapes apparently enables Clark's nutcrackers to conserve energy when hauling heavy pouch loads long distances or to high-elevation cache sites [169]. Clark’s nutcrackers can fly with pouch loads that exceed 20% of their body weight [170].

The Clark’s nutcracker is described as an optimal forager when harvesting conifer seeds—preferring to harvest pine seeds that offer the shortest search and/or handling time [137,155]. When presented with a choice between species, the Clark’s nutcracker apparently selects the option with the greatest foraging efficiency, ultimately maximizing the total number of seeds obtained [155]. Selection between preferred species apparently corresponds to cone-ripening phenology; by choosing the pine species that ripens first, Clark’s nutcrackers have the opportunity to use preferred species sequentially, prolonging their ability to store seeds [137,160] (see Seed utilization patterns).

Within a selected food species, Clark’s nutcrackers forage in a manner that that maximizes the efficiency of the seed harvest, first tending to choose populations that produce the largest seed crops and then discriminating among trees according to cone characteristics and seed crop size [24,167]. Numerous studies describe apparent selection of stands where cones ripen early [137], trees with high cone densities [16,24,155], and cones with high proportion of edible seeds [155,169]. For a further discussion of foraging efficiency, see Tomback and Linhart [156].

Harvesting—feeding and pouching: As the previous year’s seed stores become depleted in early summer, Clark’s nutcrackers usually begin foraging on fresh seed from the ripening cones of large-seeded pines. Individuals extract seeds for their own immediate consumption and/or for feeding to dependent juveniles. Foraging typically occurs singly, in pairs, or in small loose flocks [169]. Seeds from the ripening seed crop become an increasingly important part of the diet in late summer. On subalpine sites in the Sierra Nevada, Clark’s nutcrackers focus on partially ripe whitebark pine seeds by mid-July [137]. Unripe cones are hard to break open because they are tightly closed, so Clark’s nutcrackers must shred the pulpy cone scales with their sharp bills to remove seeds. Foraging takes place in tree canopies because cones remain attached to the tree. Cones are clustered at branch tips and are oriented horizontally in relation to the upswept branches, providing a stable platform for Clark’s nutcrackers to work from [137]. When foraging on unripe cones of limber pine and singleleaf pinyon (but rarely whitebark pine), Clark's nutcrackers frequently detach unripe cones and fly to a branch near the tree bole, where they hammer the closed scales open [71,169].

When eating unripe seeds, Clark’s nutcrackers expend a lot of energy and time for little energy gain [167], but they may be assessing the developing seed crop (seed density and soundness). Early foraging may alert Clark’s nutcrackers to an impending seed shortage, allowing time for relocation if necessary [169,171].

During the first 2 to 3 weeks of the whitebark pine harvest season, Clark's nutcrackers eat nearly all seeds they remove from cones [30]. Clark’s nutcrackers shift from seed consumption to seed storage as cones mature and foraging efficiency increases. As whitebark pine seeds ripen, cones turn from purple to brown and the cone material dehydrates, causing the scales to loosen and separate slightly [41,42]. Easy removal of whole seeds signals the start of whitebark pine seed storage in the Sierra Nevada [160]. When cones are ripe, Clark’s nutcrackers use well-placed bill stabs to detach the upper portions of cone scales along a thin fracture zone, exposing seeds held in the core of the cone. Ripened seeds are firm and encased in woody seedcoats, allowing the Clark’s nutcracker to extract seeds and collect them in its sublingual pouch, an indication that these seeds will be cached instead of eaten. The seed coat is usually retained when seeds are pouched [137].

Clark’s nutcrackers are highly selective when harvesting seeds for caching; they peer into and probe cones, extracting only edible seeds for the most part. A dark seedcoat is a visual cue of high seed quality in whitebark pine [41,137] and Colorado pinyon seeds [169]. Energy content of fully ripe seeds is nearly optimal; dry mass and caloric value/unit mass generally increase as seeds ripen [167]. Before "pouching" a seed, it is tested for edibility. The Clark’s nutcracker rattles the seed between its mandibles, a behavior known as “bill clicking” and also holds it between the mandibles for several seconds, a behavior known as “bill-weighing” [137,169].

Clark’s nutcrackers usually feed on pine seeds for several minutes before filling their sublingual pouches [137,169]. When foraging on singleleaf pinyon seeds, Clark’s nutcrackers consumed 5 to 15 seeds before beginning to pouch seeds [169]. Seeds are consumed as they are extracted. Clark's nutcrackers typically remove seed hulls prior to swallowing by cracking the seed between the mandibles [137,160]. Once pouching is initiated, feeding does not occur again until the pouch is emptied [167].

When Clark’s nutcrackers are transporting seeds they typically make direct, nonstop flights between harvest sites and storage areas [167], although Tomback [137] reported that Clark’s nutcrackers frequently rested in the tops of tall trees when making long flights. Caching flights comprised the most energetically demanding phase of the harvesting process [170]. Once pouches are filled, Clark's nutcrackers transport the harvested seed to storage sites. Maximum number of seeds/pouch load depends on seed size and is summarized by Tomback [145] and Lanner [71]. From observations in the Sierra Nevada, Tomback estimated a single pouch load of whitebark pine seeds consisted of 35 to 150 seeds (x=77 (SD 37) seeds; median=58; n=13) [160].

Clark’s nutcrackers occur in loose flocks during the summer and fall harvest season and also in early winter before they begin to rely on stored pine seeds [137]. Since food supplies are typically patchy but locally abundant, foraging coloniality allows Clark’s nutcrackers to concentrate harvesting efforts in areas where food is most abundant, thereby increasing foraging efficiency [34]. More individuals searching for food increases the likelihood of finding a concentrated patch of preferred seeds. In addition, colonial foraging appears to permit younger and less skilled Clark’s nutcrackers to learn about food sources from older, more proficient foragers [30]. In the Washington Cascade Range, Lorenz [85] found that emigrant Clark's nutcrackers foraged and cached in large, vocal flocks of 50 to 200 individuals; by comparison, resident Clark's nutcrackers usually foraged and cached either singly or in small groups of 2 to 10 nutcrackers.

		Cones that are worked by Clark’s nutcrackers have a distinct appearance. Frayed cone scales are an indication of harvesting activity on closed cones. Clark’s nutcrackers shred the pulpy scales with their beaks to gain access to the seeds beneath, usually beginning at the proximal end [169]. When extracting seeds from ripe whitebark pine cones, Clark’s nutcrackers typically select cone scales most easily accessed—those facing upward or sideways on the horizontally oriented cone. Because unripe seeds on the underside are often not extracted until later in the season, many cones are only partially harvested and have a hollowed-out appearance [137,142]. In comparison, rodents (chipmunks) most commonly chew off all the cone scales and leave only a core of the cone [137].
Photo courtesy of the Whitebark Pine ecosystem Foundation.

Timing, duration, and size of harvest: Timing of the autumn seed harvest is highly variable from year to year and depends upon the influence of local weather and site conditions on cone-ripening phenology [147]. An indication Clark's nutcrackers have begun harvesting whitebark pine cones is the presence of purple pitch on their bills and faces [30,137].

Dates of initial harvest, initial caching, and the end of the caching season are presented below. Site information and seed crop size for some of these studies are summarized in a review [86].

The timing of seed pouching and caching of the year's seed crop vary from year to year within the same locality and depend on many factors. Three consecutive years of observational data for whitebark pine seed harvesting on subalpine sites in the Sierra Nevada indicate the initiation of seed caching varied as much as 2 weeks between years [137].

Clark's nutcrackers harvested and cached pine seeds from closed cones in isolated mountain ranges in Nevada. In the Schell Creek Range, researchers observed limber pine caching from 9 to 11 August; in the Jarbidge Mountains, they observed whitebark pine caching from 12 to 14 September. By these dates, Clark’s nutcrackers on the 2 sites had harvested seeds from more than 50% of the trees sampled [130].

The Clark’s nutcracker harvests and stores the year’s seed crop rapidly, working intensely until the seed resource is depleted. At the height of harvest season, Clark's nutcrackers are highly active, commonly spending almost all daylight hours storing pine seed [137,167]. Hutchins and Lanner [41] observed pronounced seed harvesting and storage occurring approximately 9 hours/day (regardless of day length) on subalpine sites in western Wyoming. On hot upper-subalpine sites in the Sierra Nevada, harvest and storage usually occurred during the cooler portions of the day, typically 2 to 3 hours after sunrise and 2 to 3 hours in the late afternoon and early evening [141,160]. Vander Wall [167] also noted a decline in caching during midday for Clark's nutcrackers using hot pinyon-juniper sites in the Raft River Mountains of Utah.

The size of the Clark's nutcracker harvest depends of the size of the seed crop, with harvesting and caching occurring as long as seeds are available [41]. The effect of seed crop size on the duration of the harvest season is shown below.

During a mast year, a single Clark’s nutcracker can store large quantities of seed, estimated at 32,000 [141] to 98,000 seeds [41]. On subalpine sites the Sierra Nevada, one Clark’s nutcracker may cache an estimated 850 whitebark pine seeds/day [153]; over a harvest season, one bird may make at least 7,700 separate whitebark pine seed caches. During good seed crop years, cached seeds provide approximately 2.3 to 3 times the energetic requirements of an individual. This extra energy is required for reproduction and feeding nestlings and fledged juveniles [137,169]. Besides ensuring that at least some caches are accessible during all times of the year, excess caching apparently also provides a margin for loss due to rodent predation, seed spoilage, and “forgetting” cache locations [12,141] (see Foraging and caching).

Nonstop caching essentially saturates the subalpine landscape with seed caches in a good seed crop year. A population of 25 Clark’s nutcrackers stored an estimated 800,000 seeds within a 120-acre (50 ha) area in the eastern Sierra Nevada [141]; another population of 150 Clark’s nutcrackers stored between 3 and 5 million Colorado pinyon seeds in northern Arizona [169].

Caching: Seed caching behavior is fairly consistent throughout the Clark’s nutcracker’s range. Cache distribution within montane habitats—local and communal cache placement, distances between harvest trees and caching areas, changes in elevation between harvest trees and cache sites, and cache microsites—appear highly variable and reflect a population's use of the local habitat (see Caching habitat).

Caching behavior: Seed caching is a ritualized behavior and takes about 30 seconds/cache [51]. When caching in forest litter or mineral soil, a Clark’s nutcracker digs a shallow trench with sideswipes of its bill, inserts one to several seeds into the soil (one at a time), rakes soil and litter over the cache site, and, in many cases, places a large piece of litter (pine cone, pebbles, leaves, snow) at the site [137,166,169]. If caching in volcanic substrates, Clark’s nutcrackers may push seeds into the soil without preliminary preparations [137]. Whitebark pine seeds are buried 0.4 to 1 inch (1-3 cm); cache size ranges from 1 to 15 or more seeds, with a mean cache size of 2.6 to 5.2 seeds/cache [137]. A Clark’s nutcracker typically makes several caches in the same area, then flies a short distance to an adjacent area and makes another series of caches, continuing to cache until the sublingual pouch is empty. Tomback [137] reports nearest-neighbor distances ranging from 3.9 to 118 inches (10-300 cm) between caches within a series. See Tomback [137] for detailed information on caching behavior.

Clark’s nutcrackers typically work alone while caching [169] but may be part of flocks of up to 150 birds caching concurrently [41]. For example, on Mt Washburn in Yellowstone National Park, groups of 10 to 15 Clark’s nutcrackers cached together within a 100-m² area on several different occasions without aggressive interactions [41].

Cache distribution: The Clark’s nutcracker typically distributes seed caches in 2 ways—caching locally near source trees or traveling some distance to communal caching areas [41,62,148] (see Arrangement of seed caches).

When caching close to source trees, caches are scattered throughout nearby terrain at distances from 6.6 to 660 feet (2-200 m) [137]. In communal storage areas, however, many Clark’s nutcrackers intermix their caches, thus concentrating a high density of caches throughout the communal area. Vander Wall [167] indicates seeds cached near source trees represent only 10% to 20% of the total seeds stores of a Clark’s nutcrackers (see Caching habitat for more details on characteristics of cache sites).

Although communal storage areas may be adjacent to large stands of preferred seed trees, Clark's nutcrackers also transport seeds long distances to use communal cache sites. The distance between seed sources and storage areas is highly variable. Clark’s nutcrackers may travel up to 18 miles (29 km) to cache seeds; however, shorter distances of up to 1.2 miles (1.9 km) are more common [41,137,167]. Vander Wall and Balda [169] indicate Clark’s nutcrackers harvest seeds from trees nearest communal storage sites first; foraging distance from the communal cache site increases as ripe seeds from the closest trees become depleted. Sund [134] reports Clark’s nutcrackers tend to bury more seeds/cache when transporting seeds long distances.

Recaching: A portion of the seed cached near source trees may be stored only temporarily. Clark’s nutcrackers often make caches within 300 feet (100 m) of the harvest site and later recache seeds on communal storage slopes [41,137]. Recaching appears to increase the number of seeds harvested/individual by reducing time-consuming flights to communal storage areas. Temporary caching may allow Clark’s nutcrackers to rapidly store the available seed crop in the ground, where seeds are concealed from direct predation. As the harvest winds down, locally cached seeds can be moved, without loss of harvesting opportunities, to sites with better winter access and where seed predation is less likely [42] (see Seed cache predation). For more information on Clark’s nutcracker foraging behavior during poor seed crop years, see Migration and Irruptions and extralimital wandering.

Retrieving caches: The magnitude of Clark’s nutcracker seed caching efforts is remarkable, and the species possesses a noteworthy spatial memory that enables precise and accurate relocation of thousands of seed caches made during the previous harvest season. Clark’s nutcrackers recover cached seeds over a 6- to 9-month period extending from winter through late summer [41,137,169].

A Clark’s nutcracker remembers the precise location of its seed caches through the use of a large-capacity, well-developed spatial memory [52,139]. Large objects such as rocks, logs, and shrubs serve as visual cues during cache recovery [137,166] (see Cache microsites). Clark’s nutcrackers can accurately relocate seed caches up to 9 months after making them [12,166]; accuracy begins to decline between 183 and 285 days after caching [12]. Laboratory research indicates Clark's nutcrackers possess several levels of memory organization to recall information regarding caches. Some research suggests that a Clark’s nutcracker remembers some cache locations better than others, and these caches are among the first ones recovered [14]. Apparently Clark’s nutcrackers can track which caches have been emptied [53]. Clark's nutcrackers also remember the size of seeds within a cache [100].

Although the location of a cache is remembered in relation to other objects in the landscape, it is unclear how Clark’s nutcrackers can retrieve caches from sites that have been visually altered since cache creation—for instance, sites where snow cover removes many visual landmarks or where green vegetation dies back to ground level. Local landmarks are apparently only one factor utilized during cache recovery [13]. For a detailed discussion of research involving the memory capabilities of Clark’s nutcrackers, see Balda and Kamil [13] and Lanner [75].

When relocating a seed cache from memory, a Clark’s nutcracker first perches on a tree near the cache area and surveys the adjacent terrain, apparently selecting a site for cache retrieval. It then drops to the forest floor and begins probing a spot by thrusting its bill into the soil or by digging a trench with sideswiping motions of its bill. Once a seed is located, sideswiping behavior continues until all seeds within a cache are located and removed. Seeds are either cracked and eaten at the recovery site, leaving behind a pile of seed hulls, or pouched and transported to the nest site to feed young. Clark’s nutcrackers usually locate several nearby seed caches before moving on to a new area [137]. See Tomback [137,160] for further information on cache recovery behaviors.

Winter cache retrieval: Studies of winter cache retrieval were uncommon as of 2008. In the Sierra Nevada, Clark’s nutcrackers apparently prefer retrieving seed caches from snow-free patches or areas with minimal amounts of snow. Following heavy snowstorms, Clark’s nutcrackers rely heavily on seed caches located on steep, wind-swept communal storage slopes [160].

Winter cache retrieval apparently occurs from memory. Clark’s nutcrackers access caches by digging through the least snow possible. For instance, on high-elevation meadow sites used year-round by Clark’s nutcrackers in Wyoming, Clark's nutcrackers became totally dependent on cached seed by 2 November, when snow blanketed most of the basin. Although snow depths sometimes reached 49.2 feet (15.0 m) at this subalpine location, caches on windswept, rocky moraine ridges and south-facing cliffs remained exposed enough to allow Clark’s nutcrackers access. One Clark’s nutcracker successfully pecked through 10 inches (25 cm) of snow to retrieve a cache [41].

Winter cache retrieval has been more thoroughly studied in the European nutcracker. Crocq (1990, cited in [90]) reports European nutcrackers leave subalpine habitats when snow depths reach 70 inches (170 cm). European nutcrackers have been documented tunneling through snow to recover caches; successful efforts at cache retrieval occurred over snow tunnel distances of 50 inches (130 cm) and 120 inches (300 cm) (Stern 1994, personal communication cited in [90]).

Spring and summer cache retrieval: As weather moderates and melting snow becomes widespread, Clark's nutcrackers recover seed caches from newly exposed, wet areas near receding snow patches [137]. For example, seeds cached on sites where snow lingers (northeast-facing slopes and under the forest canopy) are used more in summer than in winter [41].

Clark’s nutcrackers sometimes continue to recover the previous year’s caches while harvesting and caching the current year’s crop [41,137]. Occasionally, recovered seeds from the previous crop are cached again, even though many may have already germinated [41]. In a review, Mattes [90] suggests nutcrackers (Nucifraga spp.) sometimes relocate and then recache the previous year’s seed caches, particularly if the current seed crop is poor.

Cache retrieval methods may be influenced by a Clark's nutcracker's migratory status. Studies of radio-tagged Clark's nutcrackers in the Washington Cascade Range found resident individuals used memory when locating caches placed within a 120-acre (500 ha) home range (Lorenz and Sullivan 2008, cited in [10]). Emigrant Clark's nutcrackers, however, located whitebark caches by searching for germinating seedlings on communal caching grounds [85].

Other foraging methods: Although Clark’s nutcrackers forage primarily on seeds in pine tree crowns, they are opportunistic and employ a variety of foraging methods throughout the year. Versatile bills enable them to probe for insects, kill small vertebrates, and feed on carrion [147]. Clark's nutcrackers glean insects and spiders from vegetation, soil, fallen pine cones, rotten logs, and stumps [147,161]. Other methods include fly-catching, bark-flaking, flying from perch to ground, scavenging, and preying upon other birds and small invertebrates [17,147] (see Diet).

During the winter and early spring, small groups of Clark’s nutcrackers forage intensely for unharvested seed in trees and on the ground. On Jeffrey pine sites in the eastern Sierra Nevada, Tomback [160] observed Clark’s nutcrackers foraging on fallen Jeffrey pine cones; Clark's nutcrackers first ate seeds that had fallen onto the ground, then ate seeds remaining in open cone scales, and finally opened any closed cone scales to remove remaining seeds.

Concerns exist regarding the reduction in seed resources available to Clark’s nutcrackers due to white pine blister rust in 5-needled pines (see Loss of habitat). Cone production has been seriously reduced in many areas where Clark’s nutcrackers depend upon whitebark pine and limber pine as primary food sources. The carrying capacity of subalpine habitats for Clark’s nutcrackers is expected to decline as the landscape becomes less able to produce sizeable seed crops at local and regional scales. Researchers speculate that Clark’s nutcrackers may move out of subalpine areas and shift foraging (perhaps permanently) to lower-elevation pine species still capable of producing large concentrations of seeds. A foraging shift leaves the potential natural regeneration capacity of whitebark pine and limber pine in doubt [154]. See the Whitebark and Limber Pine Information System [164] for stand-level distribution information for these tree species.

Clark's nutcrackers wander widely in search of seed resources. In the Washington Cascade Range, preliminary results showed that both resident and emigrant Clark's nutcracker utilization of whitebark pine and ponderosa pine seed reflected regional availability of these species [87]. These spatial patterns suggest that Clark's nutcrackers must be managed from a regional perspective to maintain Clark's nutcracker subpopulations [10].

Measurement challenges: The Breeding Bird Survey and the Christmas Bird Count survey methods have limited accuracy when dealing with a species that is nonterritorial, nests earlier than most passerines, and ranges widely in response to seed shortages [86].

In 2008, a protocol for assessing year to year variation in Clark’s nutcracker populations was being developed by the Whitebark Pine Ecosystem Foundation [177]. The Foundation encourages managers to establish permanent survey transects to serve as baselines for long-term monitoring of Clark’s nutcracker populations in areas with and without white pine blister rust infections. Baseline numbers may be difficult to establish. Population surveys of Clark’s nutcrackers over a 1-year period in the Washington Cascade Range indicated low detection rates in areas of known Clark's nutcracker residence. Of several survey methods tried, the standard point-count technique produced the highest detection rate of Clark’s nutcrackers, yielding an average of 5.5 detections in 30 minutes [88].

Loss of habitat: Clark's nutcracker habitat is declining throughout major portions of its range due to a variety of factors. Management concerns addressed below are grouped according to location of preferred food species.

High-elevation white pines: Major losses of Clark's nutcracker habitat are occurring in high-elevation communities from the widespread decline of five-needled pines [150]. Of the 5 nutcracker pines, 3 are 5-needled species susceptible to white pine blister rust—whitebark pine, limber pine, and southwestern white pine (a montane species, but grouped here for discussion) [154]. Three additional food species—Great Basin bristlecone pine, Rocky Mountain bristlecone pine, and foxtail pine—are also susceptible. Besides the ongoing spread and intensification of white pine blister rust infection, other factors contributing to mortality in these pines include mountain pine beetle (Dendroctonus ponderosae) epidemics, successional replacement due to fire exclusion, and likely effects of climate change [154]. These factors are working in concert in parts of the Clark's nutcracker's range, often with devastating results.

White pine blister rust: The nonnative fungus Cronartium ribicola is responsible for white pine blister rust infections in 5-needled pines. At high elevations, Clark's nutcracker food production and caching habitat is threatened because the high-elevation species most important in the Clark's nutcracker diet are 5-needled pines [125] (see the table in Preferred habitat). Trees infected with white pine blister rust have reduced cone production compared to uninfected trees because cankers kill cone-producing branches in the upper part of the canopy. Cone production declines many years before actual tree mortality occurs. Individual trees eventually die when cankers girdle the trunk [63]. Seedlings and saplings can also be infected with white pine blister rust [152].

As cone production declines due to white pine blister rust, researchers are concerned that Clark's nutcrackers may become more of a seed predator than a seed disperser [154]; Clark's nutcrackers may forego caching and simply consume what seed is available [57]. Although undocumented, it is generally agreed that little-realized seed dispersal of preferred pine species happens during low to moderate cone-production years [13,30,53]. Clark's nutcracker seed consumption typically comprises a greater proportion of the available seed crop when cone production is low compared to masting years.

Estimating the number of whitebark pine seeds stored and the energetic requirements of an individual Clark’s nutcracker, Tomback [141] calculated that a Clark’s nutcracker retrieves approximately 55% of its subalpine seed stores (based on seed retrieval at subalpine sites from April-July only) during a good seed crop year. During marginal seed crop years and in areas where cone production is reduced due to white pine blister rust, this percentage would probably be much higher [13,154].

Within a whitebark pine population, up to 5% of trees may possess genetic resistance to white pine blister rust [93]; these individuals form the basis of restoration efforts. Although silvicultural methods involve planting of genetically resistant stock in some areas, natural regeneration via Clark's nutcracker dispersal of genetically resistant seeds is another potential strategy for whitebark pine restoration [150]. A study of Clark's nutcracker foraging behavior in whitebark pine stands indicates a reduction in Clark's nutcracker seed dispersal from blister rust-infected stands, particularly on seral sites [95]. Implications are that even though genetically resistant trees may be producing seed, Clark's nutcracker seed dispersal may not be occurring. Consequently, white pine blister rust may potentially cause local, regional, or range-wide extirpation of whitebark pine without management intervention [94]. For a review of white pine blister rust, including its life cycle, symptoms for detecting infection, and distribution, see Schoettle and Laskowski [125].

Mountain pine beetles: Large-scale outbreaks of native mountain pine beetles are killing whitebark pine and limber pine in many locations [4,82]. Mountain pine beetles usually target large, reproductively mature trees regardless of their genetic resistance to white pine blister rust. Global warming may be contributing to an increase in mountain pine beetle activity at elevations where epidemics did not occur historically; for example, on sites over 10,000-foot (3,000 m) in elevation in the Rocky Mountains [125].

Successional replacement of seral communities: Clark's nutcracker foraging habitat has dramatically declined with fire exclusion. Historically, fire maintained the dominance of seral whitebark pine and limber pine on the landscape, providing a mosaic of foraging habitats. Absence of fire has shifted seral stands to late-successional communities characterized by closed canopies and low cone production in pine species. Current populations of whitebark pine and limber pine are likely much reduced from historical levels [55,102,108].

Climate change: There are substantial concerns regarding the persistence of high-elevation pines (whitebark, limber, Rocky Mountain bristlecone, Great Basin bristlecone, and foxtail pines) in the face of climate change. Within high-elevation landscapes, upward elevational shifts in vegetation are likely as temperatures rise. In many areas, high-elevation pines are already occupying mountaintops and ridgetops; upward expansion by lower-elevation species may reduce habitat for pure stands of high-elevation pine species. Where alpine habitat currently occurs above pine stands, Clark's nutcrackers may contribute to the upward expansion of treeline because they routinely store seeds above treeline [15,142]. Although whitebark pine seed dispersal by Clark's nutcrackers often contributes to the formation of tree islands within treeline and alpine habitats, whitebark pine mortality from white pine blister rust may occur before conifer associates are recruited [116].

Low-elevation, wingless, large-seeded pines: Fortunately, singleleaf pinyon and Colorado pinyon are not susceptible to white pine blister rust. However, in large areas of pinyon-juniper woodland in the Colorado Plateau and elsewhere in the Southwest, prolonged drought coupled with bark beetle (primarily pinyon ips (Ips confusus)) infestations have caused extensive mortality of Colorado pinyon and ponderosa pine [13]. As of 2008, it was unclear how Clark's nutcrackers were responding to this decline in foraging habitat. A situation similar to the decline in seed caching from white pine blister rust-infected stands may develop in the Southwest—Clark's nutcrackers may consume most seed produced by surviving trees, resulting in little or no dispersal of pinyon pine seeds. General recommendations for the conservation of the pinyon jay may also be appropriate for the Clark's nutcracker.

FIRE EFFECTS AND USE

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

Because Clark’s nutcrackers are early nesters and often nest on montane sites when snow is still on the ground, fire-related mortality during the nesting season (March and April) is unlikely. Clark’s nutcrackers nesting in pinyon-juniper communities may be more vulnerable to fire, since nests in these communities are only several meters off the ground [160]. Although nest height is highly variable, nests tend to be high if tall trees are available [147]. In the northern Rocky Mountains, Clark’s nutcrackers nesting in low elevation ponderosa pine/Douglas-fir communities may be more susceptible to early season fires during years of low snowpack; however, tall nest tree heights may offset this vulnerability.

While fire provides caching habitat for the Clark's nutcracker, it also eliminates seed sources that Clark's nutcrackers rely upon for food. This dual influence may be reflected in the Clark's nutcracker use of burns reported in the literature. In a review of fire effects information on western North American birds, Kotliar and others [65] report Clark’s nutcrackers demonstrated a mixed response in their use of burned habitats.

Fire effects on foraging habitat: Fires may eliminate portions of Clark's nutcracker foraging habitat by damaging or killing trees that provide seed. Extensive stand replacement fires have the potential to kill vast numbers of seed-producing trees. Historically, large stand-replacement fires probably did not pose a threat to local nutcracker populations; Clark's nutcrackers range widely, and foraging flexibility allows for a shift to alternative seed species [137]. However, it is presently unknown what effect large scale removal of seed source trees by stand-replacement fire may have in light of white pine blister rust mortality and climate change.

Foraging for seeds soon after fire: When a seed source is available, fire does little to deter Clark’s nutcrackers from foraging on pine seed. Although severe fire frequently consumes the crowns of preferred food trees, Clark’s nutcrackers often forage in tree canopies for seeds that remain in scorched cones. For example, on subalpine sites at 9,000 feet (2,700 m) elevation in central Idaho, researchers observed Clark’s nutcrackers foraging for seeds in the crowns of whitebark pine within 24 hours of a high-intensity prescribed fire (intended to mimic a stand-replacement fire) that killed 90% of the overstory. Many of the cones were still smoldering as Clark's nutcrackers worked on them. Burning occurred in early September when Clark's nutcracker seed harvest was beginning to peak [60].

Cone phenology and extent of cone heating at the time of burning probably influence the degree to which Clark’s nutcrackers utilize seeds from scorched cones. Several months following a late summer, stand-replacement wildfire that burned a low-elevation site in Montana, Hutto (2008, personal communication) [46] observed Clark’s nutcrackers foraging for ponderosa pine seeds contained in residual cones within tree crowns.

Photo by Dr. Richard Hutto, University of Montana.

Fire effects on caching habitat: Historically, late-summer fires on high-elevation sites coincided with the ripening of the pine seed crop. Since Clark’s nutcrackers tend to cache heavily in forest openings and seek out openings as they are created [153], burns presented a timely caching opportunity. Fire-caused openings generally remain available for decades after fire.

There may be several reasons why Clark's nutcrackers cache heavily in burned areas. Burns usually have minimal vegetation, provide canopy openings that allow Clark's nutcrackers access to the ground, and have objects that may help in the relocation of caches [60]. On sites within 2 areas burned by stand-replacement fire in the Northern Rocky Mountains, whitebark pine regeneration was located mostly near objects.

It is unclear whether Clark’s nutcrackers prefer to cache near or within particular plant species after fire. On sites in Montana, Tomback and others [159] found whitebark pine seedlings were more frequently associated with grouse whortleberry (Vaccinium scoparium) than whitebark pine's relative occurrence within the burn would predict. This pattern may indicate Clark’s nutcracker caching preferences, or it may indicate that whitebark pine seedlings established on sites where nearby vegetation ameliorated the postfire environment, enhancing whitebark pine seedling survival [159].

Evidence for Clark's nutcracker use of burned areas is described here in 2 sections: evidence from direct observations and evidence from successional vegetation patterns.

		Direct evidence: Clark’s nutcrackers cache in burns of varying ages. They may use burns in preference to closed forest, or they may use burns because those sites have always been used for caching. Caching soon after fire: Clark’s nutcrackers start caching in burned areas almost immediately after fire if seeds from a ripening cone crop are available nearby. For example, on high-elevation whitebark pine sites in western Montana, Clark’s nutcrackers cached whitebark pine seeds on sites burned by prescribed fire only 5 days earlier (Keane 2001, personal communication). Hutto (2008, personal communication [46]) observed Clark’s nutcrackers caching ponderosa pine seeds in a burn several months after a late-summer, stand-replacing wildfire in western Montana.
Photo by Dr. Richard Hutto, University of Montana.

Caching in postfire years 1-4: Clark’s nutcrackers continue to use burns during early postfire years. A year after the Yellowstone Fires of 1988, Clark’s nutcrackers were observed caching whitebark pine seeds on a severely burned slope of Mt Washburn; cone-bearing trees that survived the fire grew along a ridgeline immediately above the upper portion of the burn and served as harvest trees during 1989, a heavy cone crop year [151]. Within the burn, Clark’s nutcrackers cached seeds in clumps of fireweed (Chamerion angustifolium), in burned forest litter, next to a fallen tree, within a clump of charred saplings, and at the base of burned trees; clumps of grasses and sedges near the edge of the burn were used occasionally. Four years after a lightning-ignited fire in krummholz whitebark pine in the central Sierra Nevada, Tomback [142] observed Clark’s nutcrackers caching whitebark pine seed in early September; the burn was 5 acres (2 ha) in size. In the Cascade Range of Washington, 2 radio-tagged Clark's nutcrackers cached ponderosa pine seed in a low-elevation, open ponderosa pine stand that had burned in a mixed-severity wildfire 4 years earlier [87]; both individuals had home ranges that included a portion of the burn and adjacent unburned forest. Incidental observations indicate that when caching within the burn, Clark's nutcrackers frequently cached in severely burned areas (Lorenz 2008, personal communication [84]).

Caching in older burns: Clark’s nutcracker use of burns as cache sites is likely to continue at least as long as burns remain relatively open. Burns on exposed sites (climax whitebark pine or limber pine) often remain open for several centuries or longer. Researchers observed Clark’s nutcrackers caching limber pine seeds in a burn more than 90 years old during 3 consecutive years in the Raft River Mountains of northwestern Utah. Clark’s nutcrackers used the burn as a communal caching area. The burn spanned the windswept upper slope of a north-facing ridge at 8,200 to 8,370 feet (2,500-2,550 m) elevation. Slow tree growth on this harsh site helped the area retain a scattered, discontinuous cover [80].

As burns age, trees that established during early postfire years may begin to produce seed. On a century-old burn used as a communal cache site in the Raft River Mountains, Clark's nutcrackers harvested and cached limber pine seed from trees that established after the fire; the maximum age of 109 limber pines within a 1.58-acre (0.64 ha) plot was 83 years [80]. Reproductive maturity of nutcracker pines is thought to take longer to reach on harsh, high-elevation sites than on productive sites [4].

Preliminary information from studies in the Washington Cascade Range indicates Clark's nutcracker utilization of burns may not necessarily be a response to the creation of caching habitat. Studies of radio-tagged Clark's nutcrackers suggest that burns may overlay areas of existing caching habitat, and Clark's nutcrackers continue to cache in an area despite its being burned. Caching within burns varied between segments of the Clark's nutcracker population. Resident Clark's nutcrackers cached in burns only when burns occurred within their established year-round home range. Resident individuals occupying home ranges within a ponderosa pine/Douglas-fir forest had portions of their home range burned in a mixed-severity fire. Four years after the fire, residents placed less than 20% of their caches within burned patches (Lorenz 2008, unpublished data cited in [10]); Clark's nutcracker use of burned habitat occurred almost in proportion to its availability within the home range (Lorenz 2008, personal communication [84]). Although resident Clark's nutcrackers ranged widely over 4,200 acres (1,700 ha) of burned whitebark pine stands during the autumn harvest season, individuals did not cache within high-elevation burns; instead, each resident transported seed back to and cached within their individual year-round home range (n=212 cache observations). Limited radio-tracking data from emigrant Clark's nutcrackers suggest a similar pattern; emigrants tended to cache in communal caching grounds instead of high-elevation burns [10].

Indirect evidence from successional studies: Establishment of nutcracker pine seedlings provides indirect evidence that Clark's nutcrackers used a site for seed caching. Seedling establishment is a particularly valid indicator of Clark's nutcracker dispersal for whitebark pine and limber pine. Clark's nutcracker caching is the main or only means of seed dispersal for these species, although nocturnal rodents disperse limber pine in some areas [157].

Seedling establishment provides a conservative estimate of Clark's nutcracker use of a site. Calculations from direct observations of Clark’s nutcracker caching in a century-old burn in the Raft River Mountains indicated that Clark's nutcrackers cached approximately 30,000 limber pine seeds per 2 acres (1 ha) during the fall of a "moderate" cone crop year. Limber pine establishment more than 90 years after the fire was considerably less, however, at 109 individuals per 1.6-acre (0.6 ha) plot. The researchers attributed this disparity to cache retrieval by Clark's nutcrackers, cache predation by other animals, poor germination of unrecovered caches, and low survival of germinants [80].

Immediate and long-term use of seral sites: Prompt postfire seed caching by the Clark's nutcracker is important in maintaining seral whitebark pine and limber pine stands over much of the Rocky Mountains and promotes the long-term persistence of Clark's nutcracker foraging habitat. Clark's nutcrackers likely saturate burn areas with cached seed in good cone crop years. Numerous successional studies indicate that early and continuous large-scale seedling establishment promotes dominance of whitebark pine and limber pine on seral sites [21,127,159,175]. In the absence of fire, dominance by subalpine fir, Engelmann spruce, and/or mountain hemlock increases [104].

Research investigating bird community composition within severely burned conifer forests throughout western Montana indicates that Clark’s nutcrackers are more abundant in burned than unburned forest. Clark’s nutcrackers were present most often in burns less than 10 years old, followed by burns ranging from 10 to 40 years of age. Ground cover was negatively correlated with Clark’s nutcracker occurrence [44].

In the Northern Rocky Mountains, indirect evidence suggests Clark’s nutcrackers may continue caching in burned stands for 300 years or more following disturbance. Weaver and others [175] analyzed whitebark pine stand development in the whitebark pine/grouse whortleberry habitat type. Using establishment data from 47 stands ranging in age from 29 to 643 years, they found whitebark pine seedling establishment occurred at a rate of 1,000 seedlings/ha/yr, a level consistent across stands of increasing age. Studies of postfire limber pine recruitment on treeline sites in the Colorado Front Range indicate a pattern of continuous regeneration on 60- to 100-year-old burns, with seedling establishment initiated shortly after fire on most sites [127].

Although Clark's nutcrackers make immediate use of burns as cache sites, seedling establishment is frequently low. Clark’s nutcrackers apparently used burns as cache sites for at least 3 decades following wildfires in Montana. Using data from 2 burns on seral whitebark pine sites in Montana, Tomback and others [159] reconstructed whitebark pine seedling establishment 26 and 28 years after fire. Both burns displayed continuous low levels of recruitment, starting in postfire year 5 on one burn and 7 on the other. Major recruitment was episodic on both sites, however, and occurred about 17 to 25 years after fire. The researchers suggest that good cone production and favorable moisture may be responsible for the similar surge in regeneration on both burns.

Even though Clark's nutcrackers may be quick to use burns as cache sites, whitebark pine seed usually requires at least 2 winters before germination, and is germination is sometimes delayed for several years [9,151]. Because Clark's nutcrackers begin harvesting whitebark pine seeds before the cones ripen, many cached seeds contain underdeveloped embryos that mature while stored in the soil [135]. Seeds with underdeveloped embryos form a soil seed bank in whitebark pine, which is unique among nutcracker pines. No whitebark pine seedlings established during the first 5 years following a stand-replacement fire on sites in western Montana [134]; lack of regeneration during early postfire years might indicate a series of poor cone crops or unfavorable conditions for seed germination.

A study of forest regeneration in the Colorado Front Range attributed poor postfire limber pine seedling establishment to the apparent lack of Clark's nutcracker caching following fire on 2 of 5 upper-treeline sites. Although limber pine dominated the prefire community on these 2 burns, few limber pine seedlings had established 60 years after fire, even though a potential seed source existed near the burn perimeters [127].

Tree establishment patterns within large, stand-replacement burns: Some research reports Clark’s nutcrackers often concentrate seed caches in areas of a burn that are closest to source trees. Tomback and others [153] studied whitebark pine regeneration patterns on a subalpine site in Montana following a severe, stand-replacing wildfire that burned 28,050 acres (11,350 ha). A seral whitebark pine-subalpine fir stand comprised the prefire community in the upper portion of the burn. Clark’s nutcracker seed caching, as indicated by whitebark seedling densities, was most pronounced along the edge of the burn near living whitebark pines; seedling densities declined rapidly with distance into the burn. Travel time into the burn probably accounts for this pattern, with short flight distances being more efficient than long ones [144]. Studies of postfire limber pine regeneration at treeline sites on the Colorado Front Range found a similar pattern in limber pine seedling distributions [127].

Although Clark's nutcrackers frequently cache near source trees, they also cache deep within large burned areas. This habit gives Clark's nutcracker-dispersed pines a competitive edge over wind dispersed species on extensive burns. In a burned seral forest in Montana, Clark's nutcrackers cached seed at least 5 miles (8 km) into a 28,046-acre (11,350 ha) burn, where competition from wind-dispersed species was negligible. Clark's nutcrackers transported seed against the prevailing winds to sites that wind-dispersed seed was unlikely to reach [134,153].

Fire regimes: Fire creates both caching and foraging habitat for Clark's nutcrackers. The fire regimes of 11 conifer species utilized by Clark's nutcrackers are briefly described here; more information is available in the FEIS reviews of each species. 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".

High-elevation, wingless, large-seeded pines (whitebark pine and limber pine): Historical burning in both whitebark pine and limber pine communities included a variety of fire regimes that immediately produced caching habitat and eventually produced foraging habitat for Clark’s nutcrackers.

Climax stands of whitebark pine and limber pine: These stands occur on harsh sites that rarely burn because fuels remain largely discontinuous. Infrequent fires on these sites were typically nonlethal understory fires [4,5,102], which may increase caching opportunities for the Clark's nutcracker. The open tree canopy and sparse understory vegetation of climax stands generally promotes nutcracker caching without disturbance [41,160].

Seral sites with whitebark pine and limber pine: Fires on these more productive sites were generally of 2 types—mixed-severity and stand replacement. Both types of fire were infrequent: mixed-severity fires occurred at 60- to 300-year intervals [5,7] and stand-replacement fires tended to occur at intervals of more than 250 years [4,102]. Mixed-severity fires produced a mosaic of dead and live trees that apparently benefited Clark’s nutcrackers by creating caching and foraging habitat in close proximity [148]. Patches of fire-killed trees create openings in the tree canopy with areas of reduced understory vegetation—conditions that are thought to encourage caching [153]. Historically, patches of high mortality likely ranged in size from about 2 to 120 acres (1-50 ha) [58]. Stand-replacement fires created extensive areas of caching habitat for Clark’s nutcrackers. Many of the seral whitebark and limber pine stands that cover much of the upper-subalpine zone originated after large, stand-replacement fires. Stand-replacement fires often originated in lower-elevation stands and then spread into upper subalpine communities [102].

Montane, wingless, large-seeded pine (southwestern white pine): Little is known regarding historical fire frequency in southwestern white pine communities.

High-elevation, winged, small-seeded pines (Rocky Mountain bristlecone pine, Great Basin bristlecone pine, foxtail pine): Fire regime information on these species is limited, except for Rocky Mountain bristlecone pine in Colorado. In Rocky Mountain bristlecone pine communities, fire regimes are similar to those of whitebark pine and limber pine. Most climax stands are on rocky sites with sparse, discontinuous fuels, where fire does not carry well [124]. Both climax and seral stands originate after fire [11]. Because this species may be wind dispersed as well as cached by Clark's nutcrackers, the relative role of Clark's nutcrackers in maintaining postfire dominance is not clear. Lanner [74] suggests that Clark's nutcracker caching may be more prevalent on high-elevation sites, whereas wind dispersal may be more prevalent on lower-elevation sites. Understory fires may provide caching habitat by maintaining minimal surface vegetation and opening tree canopies [66].

Low-elevation, wingless, large-seeded pines (Colorado pinyon and singleleaf pinyon): Fires within pinyon-juniper woodlands may open up stands and create a mosaic of different-aged stands across the landscape [20]. A pattern of forest openings and patches of mature, seed-producing trees benefits Clark's nutcrackers by providing caching and foraging habitat in close proximity. Although pinyons generally begin to produce cones at 25 to 35 years of age, onset of good cone production occurs much later. In Colorado pinyon, good cone production occurs when trees are 75 to 100 years old; maximum seed production occurs in 160- to 200-year-old trees [118].

Low-elevation, winged, large-seeded pines (Ponderosa pine and Jeffrey pine): The ponderosa pine and Jeffrey pine types generally experienced a nonlethal understory fire regime characterized by frequent, low-severity fires [6]. With fire exclusion, these types have experienced altered fire regimes. Present-day fuel accumulations often result in severe, stand-replacement fires that consume the crowns of mature, cone-producing trees. In view of the large acreages dominated by these species, Clark's nutcrackers presently have a dependable seed resource throughout much of their range. Clark's nutcracker access to these species is important where stands of ponderosa pine and Jeffrey pine are near nutcracker pines and in years of low cone production by nutcracker pines [137].

Low-elevation, winged, small-seeded conifers (Douglas-fir): Underburns characterized dry Douglas-fir forests, and mixed-severity fire regimes typically occurred on more moist sites. Mixed-severity fires were characterized by low- to moderate-severity underburns at relatively frequent intervals (7-20 years) and severe crown fires at longer intervals (50-400 years) [5,64]. Despite the small size of Douglas-fir seeds, Clark's nutcrackers tend to use them on a consistent basis, especially in the Northern Rocky Mountains, the Cascade Range, and other portions of their northern range [38,85,86,147].

The Fire Regime Table summarizes characteristics of fire regimes for vegetation communities in which Clark's nutcracker may occur. Follow the links in the table to documents that provide more detailed information on these fire regimes.

Fire history studies in the Pacific Northwest indicate many whitebark pine forests both in the Cascade and Pacific Coast ranges had a mixed-severity fire regime [128]. Murray [107] suggests that managers should prioritize fire reintroduction in seral stands characterized by historically frequent, nonlethal surface fires. He adds that every effort should be made to protect white pine blister rust-resistant trees from lethal burning. If wildfires are approaching the whitebark pine type, managers can locate blister rust-resistant trees out ahead of the fire (safety permitting) and remove fuels around resistant trees [107].

Management considerations emerging from a whitebark pine restoration project: In northern Idaho and western Montana, researchers have conducted a number of prescribed fires as part of a long-term study aimed at whitebark pine restoration. The high-elevation, seral whitebark pine stands selected for treatment were located within subalpine fir/smooth woodrush (Luzula hitchcockii) habitat types. Prescribed fire treatments with and without fuel enhancement attempted to mimic historical mixed-severity fire regimes, with the goal of optimizing Clark’s nutcracker caching habitat. Treatments that produced Clark’s nutcracker caching included stand-replacement fires, mixed surface and crown fires, and a thin-and-burn treatment—a silvicultural cutting created "nutcracker openings" within a thinned matrix, followed by a mixed-severity fire. See Keane and Parsons [60] for details on fuels and fire prescriptions, behavior, and effects of these treatments.

Treatments using stand-replacement fires: Fire prescriptions for sites on Blackbird Mountain on the Salmon-Challis National Forest of Idaho aimed to produce conditions that: 1) mimicked stand-replacement fire and 2) would encourage abundant Clark’s nutcracker caching within the burn treatment. The site exhibited little detectable white pine blister rust infection or tree mortality, and was burned under prescription on 11 September 1999. Clark’s nutcrackers cached extensively immediately following severe prescribed burning on 2 sites (with and without fuel enhancement treatments). Whitebark pine trees in a nearby open-canopy, old-growth stand provided a local seed source for caching. Burn site characteristics: 9,000 feet (2,700 m); 10% to 20 % slope; southwest-facing slope; late seral stage composed of 200- to 300-year-old whitebark pines [60].

Treatment using mixed-severity fires with fuel enhancement: Fire prescriptions for Bear Overlook on the Bitterroot National Forest of Montana aimed to mimic a mixed-severity burn by killing >50% of the overstory and >50% of understory subalpine fir, thus creating optimal caching habitat for Clark’s nutcrackers. The fire prescription called for fuel enhancement to produce a moderate-severity fire. Although subalpine fir and Engelmann spruce were cut to increase fuel continuity, variable fuels produced a patchy burn. Clark’s nutcrackers were not observed caching on these sites (Keane 2008, personal communication). Burn site characteristics: 7,200 feet (2,200 m); mostly flat (<10% slope); southeast-facing slope; 200-acre (80.9 ha) treatment; incidence of white pine blister rust not given [60].

Nutcracker openings--treatments using silviculture and fire: Researchers designed silvicultural cuttings combined with prescribed burning to mimic patchy, mixed-severity fires. Cuttings created nutcracker openings within a thinned matrix on 2 sites; nutcracker openings were different sizes between sites [60]. Results varied:

Crews burned the units in October 1996 with a moderate-severity prescribed fire; post-frontal combustion lasted approximately 7 days. Although the cone crop was poor during 1996, Clark’s nutcrackers cached extensively in the fall of 1997 and 1998, caching in both nutcracker openings and underburned areas (Keane 2008, personal communication). See the Fire Case Study in the FEIS whitebark pine review for details on fuels and the fire prescription, behavior, and effects of these treatments.

Beaver Ridge (Clearwater National Forest, Idaho): Treatments included a noncommercial cutting that created openings in the tree canopy where Clark’s nutcrackers could access the ground and cache seeds. Nutcracker openings were evenly distributed throughout the units and consisted of 1- to 3-acre (0.4 to 1.0 ha) openings where all trees except healthy whitebark pine were cut and left where they fell, delimbed, and the slash either piled or scattered. Between the nutcracker openings, all subalpine fir and Engelmann spruce were cut, delimbed, and the slash either piled or scattered; all whitebark pine and lodgepole pine trees were left standing. Two units were successfully burned under prescription the same day, with low- and moderate-severity fires. Although the combination of cutting and burning opened the canopy and removed shade-tolerant competitors, Clark’s nutcrackers did not utilize these burns. Lack of a local seed source may be responsible for the absence of Clark’s nutcracker caching. As of 2008, many of the whitebark pine leave trees had died of fire-related causes following burning, and the local population of whitebark pine is somewhat isolated and infected with white pine blister rust [60].

General results: Most mixed, moderate- to high-severity prescribed fires produced Clark’s nutcracker caching habitat by opening late-seral stand canopies. Although Clark’s nutcrackers cached extensively on most sites soon after prescribed burning, whitebark pine regeneration remained low 5 years after burning (and 10 years after burning on one site). Researchers suggest that Clark’s nutcrackers retrieved all cached seeds on these sites, with few or no caches remaining to produce seedlings [60] (see White pine blister rust).

Management recommendations: In treatments where cutting is used to open up stands and encourage Clark’s nutcracker caching, slash can be removed by either broadcast burning or piling and burning. It may require 10 to 20 years to assess the efficacy of prescribed burning to promote whitebark pine regeneration on upper-subalpine sites. Natural whitebark pine regeneration via Clark’s nutcracker caching depends upon the availability of an adequate seed source. Planting with white pine blister rust-resistant seedlings is recommended on sites where white pine blister infection is above 50% and whitebark pine mortality is above 20% [60].

Management considerations regarding white pine blister rust: Although prescribed fires combined with silvicultural treatments may create caching habitat, Clark's nutcrackers may not use the sites for caching. McKinney [95] found Clark’s nutcracker occurrence was strongly related to cone production in the northern Rocky Mountains; in turn, cone production was positively correlated with live whitebark pine basal area and negatively correlated with whitebark pine mortality and level of white pine blister rust infection. He suggests a minimum of approximately 1,000 cones/ha must be present for Clark’s nutcrackers to initiate seed pouching and caching in an area; stands with an average whitebark pine basal area of 5.0 m²/ha meet this requirement. Unfortunately, cone production and basal areas of many whitebark pine stands in the most northerly portions of the Rocky Mountains are presently below these levels, so whitebark pine restoration via "natural" regeneration from Clark’s nutcracker seed dispersal may not be a viable option in areas of pronounced whitebark pine decline [94].

Whitebark pine seed dispersal in white pine blister rust-infested stands may be diminished by red squirrel and Clark’s nutcracker seed consumption. Using data from forest sites in the Bitterroot Mountains of Montana and Idaho, researchers found high cone-depletion rates associated with high white pine blister rust damage in both climax and seral stands [94]. On sites with high infection rates, not only did trees produce fewer cones/ha initially, but seed consumption by Clark's nutcrackers and red squirrel cone cutting further reduced cone densities [94].

McKinney and Tomback [94] suggest that in response to reduced whitebark pine cone densities in seral stands, red squirrels may move into climax whitebark stands, areas where Clark's nutcrackers historically harvested most of the cones. See Pine squirrel competition for cones for further information on this topic.

Management considerations regarding climate change: Climate change is generally regarded as a major influence driving recent increases in wildfire frequency, severity, size, and duration in the western United States [176]. Many high-elevation forests of nutcracker pines have been shaped by mixed-severity fires that created complex mosaics of live and dead trees across the landscape, a situation conducive to Clark's nutcracker foraging and caching habits. This mosaic may be eliminated as wildfires increase in size and severity. Although stand-replacement fires on seral sites have provided Clark's nutcrackers with sizeable, long-term caching areas, large-scale fires may eliminate the seed source for future stands and isolate surviving seed producing stands to the point where Clark's nutcracker dispersal is unlikely. Instead of large, stand-replacement fires being rapidly colonized by seral stands of nutcracker pines, late-seral species with seeds that Clark's nutcrackers do not use may gradually occupy the burned area via a filling-in process from the edges of the burn [127]. Extensive fires may also spread into climax stands of nutcracker pines, which historically experienced long fire-return intervals of 400 years or more [66].

For more detailed suggestions on the fire management of tree species important to the Clark’s nutcracker, see the FEIS reviews of whitebark pine, limber pine, southwestern white pine, Colorado pinyon, singleleaf pinyon, Jeffrey pine, ponderosa pine, Great Basin bristlecone pine, Rocky Mountain bristlecone pine, foxtail pine, Douglas-fir.

APPENDIX: FIRE REGIME TABLE

Nucifraga columbiana: REFERENCES

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