Food Habits

Main Foods Taken

Usually mammals, from small rodents to large hares; also birds, ranging from small songbird nestlings to medium-sized geese. Fish and other small aquatic animals less often. Probably opportunistic feeders, but likely specialize when local prey are abundant.

Microhabitat For Foraging

In arctic, areas occupied by rodents, notably lemmings. Sites are conspicuous, pockmarked by the holes of lemming burrows which in summer are excavated in turf (often beneath rocks); in winter, through layers of snow. At openings where burrows come to the surface, lemmings are especially vulnerable to the owls. In desert tundra where turf is scarce, the vegetation with its lemming burrows is concentrated at hummocky or rocky prominences. Owls using these sites in turn fertilize the surrounding nitrogen-poor soil. In time these sites become luxuriant green spots in an otherwise austere landscape (Parmelee et al. 1967)—important microhabitats not only for plants, lemmings and owls, but also for weasels, foxes, and the natives who trap them. Not known if the owls ever maintain a lemming diet during a polar winter.

On winter grounds, the owls select habitats where prey is most available (Boxall and Lein 1982a). Likewise on the breeding grounds, the owls choose areas with high lemming densities. Owl density varies not only in different valleys on Baffin Island, but also in different parts of one valley (Watson 1957), as well as other areas in the Canadian Arctic (DFP).

Food Capture And Consumption

Appear to have extraordinary vision in locating and catching prey. Equally adept at hearing, judging by birds’ ability to locate lemmings in turf and snow. These attributes need to be tested. Virtually nothing is known about their sense of smell, or other means they may have in locating prey. Commonly pursue prey through flight and capture in the air, on the ground, and on the water. Also pounce on prey from a perch, or while standing or walking. Several times Audubon (1840) observed a Snowy Owl catching fish while lying lengthwise belly down on a rock beside a water hole. Small fish were devoured near the hole, larger ones carried off.

Of 51 hunts observed in s. Alberta, “sit-and-wait” technique was witnessed in 50, hovering only once; overall success rate (capture) 43%, adult females significantly more successful than juvenile females (Boxall and Lein 1982a). Juveniles also inept at handling prey. Watson (1957) frequently observed hovering on the breeding grounds and concluded that breeding males have no difficulty catching prey when the latter are abundant.

On Baffin Island, Snowy Owls hunt in all weather and at all hours during the continuous summer light, although they are seemingly less active about noon and midnight (Watson 1957). On Greenland, they hunt mostly in early morning and late afternoon (Manniche 1910). More observations on foraging times are needed for both breeding and nonbreeding periods. Not known if these owls hunt at night, or even by moonlight, during winter darkness.

Hungry owls eat lemmings and mice head first, swallowing them at one gulp (Watson 1957). After a few have been eaten whole, others are carefully picked and eaten in small pieces, starting at the head. Commonly before eating, owls look at prey, raising the head higher and standing high on the feet before making the first bite. After feeding, both sexes sometimes wipe their bills and faces. Snow is used often in cleaning (P. Kerlinger pers. comm.).

Major Food Items

Lemmings (varying and/or brown), where and when abundant in the Arctic (Gabrielson and Lincoln 1959). At times they appear to be exploited exclusively (Sutton and Parmelee 1956, Watson 1957), or nearly so (Parmelee et al. 1967). Murie (1929), however, stated that this owl’s food varied with the character of the nesting ground, rodents being taken almost exclusively in the uplands while waterfowl were taken to some extent in the marshy areas. Seven nesting pairs observed by Dufresne (1922) suddenly switched from rodents to ptarmigan when the latter became abundant after hatch.

When lemmings are scarce or absent, a wide variety of prey is consumed. During a nine-year study in the Shetland Islands, rabbits were the preferred prey, but during rabbit lows waders and other birds formed a major part of the diet (Robinson and Becker 1986). Nonbreeding individuals may subsist in areas that lack mammals entirely. On Agattu Island in the Aleutians, pellet analysis showed that Ancient Murrelets (Synthliboramphus antiquus) composed 68% of the diet, other alcids 6.5%, and ducks 13.4% (Williams and Frank 1979).

Quantitative Analysis

On the wintering grounds, a wide variety of live prey, also the remains of dead. In sw. British Columbia, grebes and ducks were 80% of the owls’ diet with the Horned Grebe (Podiceps auritus; captured on the water) most prevalent (Campbell and MacColl 1978). Prey selected by size within a range of 400–800 g body weight. Where mammals abundant in Maine, rats and mice comprised 35%, snowshoe hares (americanus) 20%, and passerine birds only 10% of diet (87 stomach samples; Mendall 1944).

In s. Alberta, deer mice (Peromyscus maniculatus) were the most abundant species in 100 pellets collected during 1976–78 (Boxall and Lein 1982a). Meadow voles (Microtus pennsylvanicus) were only half as numerous, but the two rodents comprised about 90% and 75% of the total prey individuals. Lesser prey included ground squirrels, weasels, jackrabbits, a few passerines, and partridge. Biomass calculations, however, showed that mice and voles together formed only 62% of the calculated prey biomass in 1976–77 and only 28% in 1977–78. Richardson ground squirrels (Spermophilus richardsonii) formed 14% and 28% of calculated prey biomass in the two seasons respectively, while during one winter Gray Partridge (Perdix perdix) alone comprised 33%. Also in Alberta, Kerlinger (pers. comm.) observed that owls were abundant where partridge were abundant. Dietary differences between seasons were thought to be related to differences in weather.

Males prey almost exclusively on rodents, females on a wider range of prey, including larger species (Boxall and Lein 1982a). Diet of different age cohorts not to differ greatly.

Lemming density in the most representative type of tundra at Barrow, AK, varied from one per 2.47 ha to 42 per 2.47 ha (or 0.04 to 17.0 lemmings per ha) three years later (Thompson 1955). During a peak lemming year on Baffin Island, Watson (1957) found the proportion of lemmings eaten to be far less than might be imagined. Taking into account that peak lemming populations increase from early to late summer, the daily intake of three adults was less than 0.1% of the lemming population. Even in August, 3 adults plus 15 young still consumed less than 0.2% of the lemming population daily.

Watson (1957) estimated that adults eat 3 to 5 full-grown lemmings daily, or the equivalent of 600 to 1,600 lemmings per year. This amounts to 150–350 g per day or 55–130 kg per year. From early May to early Sep, a pair would probably eat 400 to 1,100 full-grown lemmings; including their brood of nine owlets, the total number of lemmings eaten in their territory would amount to 1,900 to 2,600. Males appear to need more food than females during nesting, presumably because of their more active role in securing food while the females incubate and brood.

Later studies (Gessaman 1972) indicate that Watson’s estimate was low with respect to the daily food requirements of a free living bird, especially during winter where ambient temperatures can drop below –40°C. Gessaman found that the daily requirements for a confined owl are 200–400 g of lemming at air temperatures between –4.5°C to –40°C. The average gross energy intake of birds caged outdoors when the average air temperature was –34°C was equivalent to 400 g of lemming. Wild owls with more activity should require more than 400 g/d.

Less is known about daily and annual intake beyond the range of lemmings where a large variety of foods may be consumed. Beamer (1937) estimated that a wild owl would eat 3,000 to 5,000 Microtus in a year, amounting to 60–150 kg.

No quantitative studies. Captive birds drink water, especially after feeding (Watson 1957). Not known if owls can substitute snow for water. Adults and young eject pellets after stretching head and neck and wide gaping of the bill. Although nest is fairly clean throughout the egg-laying period, it becomes increasingly soiled following hatching of young, not only through defecation and pellets, but also through a surplus of decaying lemming carcasses. Vegetation if present is much reduced, but the richly manured ground is rapidly recolonized, judging by layers of pellets from different nestings (Watson 1957). The pellets contribute to the bulk and composition of the turf.

Needs further study. Given the opportunity, some nesting owls switch diet from rodents to ptarmigan when the latter become available. Captive birds select mice over rats (Watson 1957). Will cache surplus lemmings at perches apart from nests (as many as 26 uneaten carcasses have been found; Parmelee 1972), but function of perch cache is not clear. Perhaps male must procure a certain number of lemmings in order to bring the female into breeding condition.