Food Habits

Fig. 2. Gyrfalcon in pursuit of Willow Ptarmigan, staple prey for this species

Feeding

Main Foods Taken

Mostly birds and predominately ptarmigan, passerines to geese; some mammals, microtines to hares (Lepus spp.).

Microhabitat For Foraging

Most prey taken on or near ground.

Food Capture And Consumption

(From White and Weeden 1966, Platt 1977, Cade 1982, Jenkins 1982, White and Nelson 1991, Garber et al. 1993, Dekker and Lange 2001.) Three main methods of finding food: (1) perching at a spot with a commanding view; (2) quartering terrain at low altitude with flapping and gliding flight; (3) soaring along ridges or over valleys, not usually at high altitude, similar to Golden Eagles (Aquila chrysaetos). Four methods of pursuing prey: (1) on ground; if prey spotted at a distance, falcon flies close to ground using terrain to conceal approach and take prey by surprise; (2) tail-chasing (Fig. 2); failing to achieve surprise, falcon pursues prey over long distances, forcing it to ground or aloft to exhaust it; (3) hovering; if prey is in cover, falcon attempts to flush it into flight by making short stoops; (4) direct climb; to gain altitude on birds with light wing-loading and better soaring abilities, falcon flies up at steep angle, rather than “ringing” up, as does Peregrine Falcon.

Method of taking prey: either on ground or by short stoop; prey more likely to be struck or driven to ground than grabbed in air. Kills typically have broken sterna. Gyrfalcons, especially immatures, sometimes pirate food from other raptors. Little information on successful capture rate, but wintering birds observed successful in 10-28% of chases after feral pigeons (Dekker and Lange 2001, Dekker and Court 2003).

Manner of consumption: poor representation of cranium and distal wing elements at nest indicates that, during nestling season, prey are decapitated and distal portion of wings removed at kill site. Large amounts of plumage at kill site indicate that medium to large birds are plucked prior to transport to nest site (Langvatn 1977). Booms and Fuller (2003a) found 96% of ptarmigan delivered to video-monitored nests in Greenland were plucked, most of them completely so. Most ptarmigan delivered to nest included the breast and back, sometimes the legs and wings, and uncommonly the viscera. Feeding bouts on ptarmigan at the nest averaged 16 min (range 1-30 min). Ptarmigan brought to females during courtship relatively unprepared (Platt 1977).

Young (leveret) Arctic hares (Lepus arcticus) typically delivered without fur removed (86% of deliveries); if > 600g, delivered in pieces (Booms and Fuller 2003a). Average feeding bout on a leveret at the nest 10 min (range 1-26 min). Adult arctic hare transported in sections (Poole and Boag 1988). Microtine rodents and passerines not decapitated or “plucked” (Langvatn 1977, Booms and Fuller 2003a). Sternum, forelimbs, primaries, secondaries, rectrices comprise > 80% of prey remains (i.e., uneaten portion) at nest site. Hind limb elements, vertebrae, mammalian and passerine bones predominate in pellets (i.e., portion consumed) (Langvatn 1977). Vegetable matter and grit in pellets indicate that some viscera of avian species are eaten (Langvatn 1977, Nielsen and Cade 1990b), but apparently viscera of ground squirrels (Spermophilus spp.) are not (Platt 1977, Poole and Boag 1988). Pellets at male perch sites during breeding season contain mainly beaks, claws, and gizzard linings, suggesting that these may be differentially consumed while other parts are fed to young (C. M. White pers. comm., TLB). Bones in pellets highly fractured and modified by digestion; few complete bones and those from the axial skeleton and especially the head are rare (Bochenski 1998).

Delivery of prey fairly uniform throughout the day during the nesting season (Fletcher and Webby 1977, Bente 1981, Poole and Boag 1988), though Jenkins (1982) and Booms and Fuller (2003a) found delivery rates peaked in late morning and evening and declined sharply between 24:00 and 04:00 h. No information on timing of foraging during the nonbreeding season. Retrieval time of 6 prey deliveries observed from a nest site averaged 15 min/prey item (Platt 1977). Length of hunting sorties observed from helicopter ranged between 22 and 67 min (n = 14); average time for female 27 min, male 38 min (White and Nelson 1991). Larger prey associated with longer foraging trips (Poole and Boag 1988). No cooperative hunting known.

Diet

Main Foods Taken

Almost without exception, Gyrfalcons rely heavily on ptarmigan across their circumpolar range and throughout the year; numerous diet studies have repeatedly documented ptarmigan contribute the majority (50-95%) of total biomass eaten. Other birds taken range between 0.02 kg and 4 kg, including primarily waterfowl (Anser spp. and Anas spp.), seabirds, shorebirds (Scolopacidae), and passerines (Passeriformes). Other documented avian prey include sage grouse (Centrocercus urophasianus), gulls (Larus spp.), fulmars (Fulmarus glacialis.), terns (Sternus spp.), Black-legged Kittiwake (Rissa tridactyla), jaegers (Stercorarius spp.), alcids (Alcidae), Rough-legged Hawk (Buteo lagopus), falcons (Falco spp.), Ring-necked Pheasant (Phasianus colchicus), Short-eared Owl (Asio flammeus), ravens and crows (Corvus spp.), magpie (Pica sp.), Savannah Sparrow (Passerculus sandwichensis), Lapland Longspur (Calcarius lapponicus), Snow Bunting (Plectrophenax nivalis), redpoll (Carduelis spp.).

Gyrfalcons feed more on resident than migrant species. Mammals ranging from 0.01 kg to 4.5 kg, including primarily hares (Lepus spp.), ground squirrels (Spermophilus spp.), and lemmings (Lemmus, Dicrostonyx) but also documented are arctic fox young (Aloplex lagopus), shrews (Sorex spp), and voles (Microtus spp, Clethrionomys spp) (see references in Appendix 1; also Cramp and Simmons 1980 and Cade et al. 1998 for species taken in Palearctic).

Domesticated species taken rarely (81 attacks over 161 yr, Tømmeraas 1988), mostly chickens (Gallus domesticus) and Rock Doves (Columba livia) (Dekker and Lange 2001) but also domestic geese, ducks, rabbits, a turkey, and a cat; taken primarily by young birds in Palearctic region, where humans and falcons are in closer proximity. Some eating of carrion (Kuyt 1980, Palmer 1988, Tømmeraas 1989), probably an adaptation to a harsh climate (Tømmeraas 1989). Will eat meat left by humans specifically for feeding Gyrfalcons (Randklev and Randklev 1994, Nielsen 2002).

Quantitative Analysis

See Appendix 1. Although Gyrfalcons take a wide variety of prey, individual birds or pairs exploit relatively few species. Willow (Lagopus lagopus) and Rock (L. mutus) ptarmigan are the dietary mainstay for most birds during the breeding season, but there are differences in diet relative to habitat (Nielsen and Cade 1990a, Huhtala et al. 1996). Coastal pairs take more waterfowl and seabirds and fewer ptarmigan, and pairs at higher latitudes and elevations take more mammals than do pairs in other habitat types.

Diets in Appendix 1 determined by prey remains and pellet analysis, which often present a biased view of diet (Marti 1987, Booms and Fuller 2003b). In central w. Greenland, prey remains and pellets overestimated ptarmigan and underestimated arctic hare occurrence at Gyrfalcon nests. Remains underestimated while pellets overestimated passerine occurrence in the diet (Booms and Fuller 2003b). Therefore, summaries in Appendix 1 should be reviewed with these potential biases in mind.

Diet shifts occur seasonally within a given habitat type. Adult ptarmigan taken most heavily early in breeding season, with proportion of alternative prey (shorebirds, waterfowl, passerines, and/or mammals) increasing later (Poole and Boag 1988, Nielsen and Cade 1990a, Booms and Fuller 2003b ). Gyrfalcons nesting in upland habitat where migratory bird and resident rodent populations are low experience least seasonal variability, relying heavily on ptarmigan year-round (Nielsen and Cade 1990b). But even these birds may experience seasonal diet shifts; in Yukon, Gyrfalcons feed on Rock Ptarmigan during breeding season and on Willow Ptarmigan during winter (Platt 1976). Birds remaining on territory may experience diet shifts as a result of changing hunting habitat (Nielsen and Cade 1990a). Because habitat influences diet, birds that abandon territories during winter probably experience diet shifts; limited observations of foraging birds outside their breeding range indicate tendency to feed on species that congregate in significant numbers such as waterfowl, game birds, and feral pigeons (Dobler 1989, Garber et al. 1993, Sanchez 1993, Dekker and Lange 2001, Dekker and Court 2003).

Annual variation in diet may occur where prey species exhibit large population fluctuations, but not in all locations. Some ptarmigan populations are known to exhibit cyclic changes in numbers (Mossop and Hayes 1994). Lemmings are heavily used in ne. Greenland in years when rodent numbers peak, and the diet shifts to passerines when rodent numbers are low (Gilg et al. 1997). When rodents are abundant, Gyrfalcons may feed heavily on rodent predators as well (Cade 1960). However, Gyrfalcons in Sweden did not shift diet when microtine rodents peaked in abundance (Nyström et al. 2006). Weather may also influence annual distribution, phenology, and availability of prey species such as ptarmigan and ground squirrels (Poole and Boag 1988, Nielsen and Cade 1990b).

Food Selection And Storage

Prey selection may be motivated more by vulnerability of prey than abundance. In Alaska and Iceland, Gyrfalcons take displaying male ptarmigan preferentially when non-flocking behavior, courtship displays, and molting plumage make them more vulnerable, even though they are not the most abundant prey species. At end of season, young ptarmigan of year are taken preferentially (Cade 1960, Nielsen and Cade 1990b). In Alaska, Iceland, Northwest Territories, and central w. Greenland, a switch from ptarmigan to other species coincides with decreased vulnerability (but not abundance) of ptarmigan, arrival of migrant species, and emergence of mammalian species (particularly juveniles) (Cade 1960, Poole and Boag 1988, Nielsen and Cade 1990b, Booms and Fuller 2003b). Both sexes take same size range of prey, but average size of prey brought by male is smaller, owing to greater proportion of passerines and small mammals (Platt 1977, Poole and Boag 1988). Immatures may preferentially take rodents and passerines (Bird and Bird 1941, Cade 1982). The relative proportion of Rock Ptarmigan (compared to Willow Ptarmigan) in Gyrfalcon diet in Sweden was positively correlated with the relative amount of Rock Ptarmigan habitat present in breeding territories (Nyström et al. 2006). Rock Ptarmigan were also overrepresented in the diet and this was interpreted as a potential preference for Rock over Willow Ptarmigan.

Females perform 93–100% of caching during breeding season. Stored food usually placed behind vegetation within 100 m (maximum 200 m) of nest site (Poole and Boag 1988). Cached prey often retrieved and fed to chicks or consumed between regular feedings when chicks can feed themselves (Platt 1977, Jenkins 1978, Bente 1981, Poole and Boag 1988). Caching occurs between chicks hatching and reaching 43 d of age, being greatest when chicks are small, and thus prey is not completely consumed in a single feeding. No caching of microtines or passerines (Poole and Boag 1988). Removal of prey remains from nest variable; Platt (1977) documented no such behavior, Booms and Fuller (2003b) found 21% of food remains removed from nest. No information on caching by males, though 10% of cached items retrieved and delivered to the nest were by males (Booms and Fuller 2003b). Little information on caching outside breeding season; one observation of a cached frozen ptarmigan being retrieved and “chipped” apart during mid-winter in the Aleutian Is. (C. M. White pers. obs.), an immature female cached part of Mallard at the base of a routinely used perch tree on Skagrit Flats, and a female cached part of Ring-necked Pheasant at the base of a barbed wire fence post in California (B. Walton pers. comm.).

Nutrition And Energetics

Little information. Research by Barton and Houston (1993) on comparative digestive efficiency of raptors would suggest that an opportunistic species such as Gyrfalcon should have relatively high digestive efficiency on a wide range of species with variable nutritional quality. An estimated 1.0–1.5 kg of food/d (1.7–2.7 ptarmigan/d) are needed during the breeding season for a family of Gyrfalcons, for a total of about 75–110 kg—the equivalent of 150–200 ptarmigan—for the entire breeding season (courtship through fledging) (Cade 1960, Bengston 1971, Pulliainen 1975, Poole and Boag 1988). During the nestling period, Gyrfalcons delivered 99, 82, and 54 kg of food to three video-monitored nests, with 4, 3, and 2 young, respectively (Booms and Fuller 2003b, 2003c). These represent estimates of 106, 94, and 110 kg of food delivered between hatch and fledging to each nest. Based on direct nest observations, Tømmeraas (1994, cited in Cade et al. 1998) estimated a pair with 4 young eats 71 kg of food during the nestling period. Hence, theoretical calculated estimates above may be biased low or birds studied by direct observation delivered more food than needed. Prey biomass per time spent foraging is higher for larger species, so small prey may only be profitable when they can be obtained quickly (< 10 min) (Poole and Boag 1988). Adult male Gyrfalcons commonly seen capturing fledgling passerines within 500 m of nests (TLB).

Metabolism And Temperature Regulation

No quantitative information, but see Breeding: young birds, growth and development; and parental care, brooding. Plumage generally softer and less compact than that of other falcons. Down highly developed and tarsus densely feathered on more than upper half, with some scattered feathers lower down (Cade 1982).

Drinking, Pellet-Casting, And Defecation

Drinking rarely observed in the wild, but captive birds drink; adequate water probably contained in food under most conditions. Pellets are long and oval, 2.2 cm x 5.0 cm on average; consist of feathers, fur, small to medium-sized bones, and occasionally vegetable remains from digestive tracts of ptarmigan (Langvatn 1977, Nielsen and Cade 1990b). Probably cast daily, but observations on captive birds indicate that frequency of casting is determined by amount of casting material consumed. No information on rates of defecation.