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Main Foods Taken
Capable of killing a wide range of prey species from small to large mammals and birds during any time of year. One of the first attempts to review diet comes from Fisher (1893). In a comparison of seven North America studies reporting >100 winter prey items, Detienne et al. (2008) reported 14 species of mammals, 43 species of birds, and one mussel as prey (Table 1).
During the breeding season in North America, Snowy Owls are known to depend on high densities of small mammals such as lemming or voles, with few exceptions (Parmelee 1992, DWH). In n. Europe, Scandinavia, and Russia, similar dependency on small mammals also reported (see Cramp 1985, Potapov and Sale 2012). Can also be an opportunistic feeder and concentrate where local prey is abundant (Bent 1938, Boxall and Lein 1982a, Mikkola 1983, Robinson and Becker 1986, Detienne et al. 2008).
Will also eat carrion such as walrus, seal, fox, fish, or dead animals caught in traps (Voous 1988, DWH, N. Smith pers. comm.). Insects and crustaceans also found in pellets (N. Smith, pers. comm.). Has been observed scavenging recently killed Caribou (Rangifer tarandus) and bringing pieces to a nesting female (DWH). During the Arctic winter, may scavenge Polar Bear (Ursus maritimus) kills. Importance of scavenging to the overall diet needs further study.
Microhabitat For Foraging
No quantitative data. Foraging areas have been qualitatively described as open habitats, e.g., agricultural fields, coastlines, grasslands, marshes, shorelines, tundra, and marine environments, including offshore islands, leads in the pack-ice, and polynyas.
Within these habitats the owls appear to concentrate foraging efforts in areas of local prey abundance (Boxall and Lein 1982a, Detienne et al. 2008). For example, during the non-breeding season, Snowy Owls will hunt in marshes with high densities of waterfowl and shorebirds, or in grasslands with high densities of small mammals or game birds. In the Arctic, Eskimo seal hunters have reported Snowy Owls to hunt wintering sea birds and sea ducks within open areas of ice. Also believed to winter at polynyas far out to sea, where masses of wintering seas sea ducks and alcids occur (DWH). Likewise on the breeding grounds, the owls choose areas with high lemming densities, but will supplement their diet with birds, ground squirrels, and hares.
Food Capture And Consumption
Hunts any time of day, except in severe weather conditions. Foraging activity is dictated by individual hunger, season and, if breeding, by the nutritional demands of the incubating female and nestlings. When hunting individually, individuals appear to prefer evening into darkness and early mornings. During 24 h of Arctic light, appears to hunt more actively during theoretical night (DWH). Less is known about nocturnal hunting, but during the Arctic winter the birds must hunt in the dark and winter observations using night vision equipment clearly show an active nocturnal hunting period (N. Smith, pers. comm.). See also Manniche (1910) and Watson (1957).
Leading edge barbs of flight feather p10 is reduced in length and density, and the surface area of barbules is also reduced, unlike the congeneric Great Horned Owl. This could indicate silent flight and hearing are less important for hunting than in the Great Horned Owl, and that vision and movement guide hunting Snowy Owls. During attempts to trap wintering Snowy Owls, the owls apparently see a bait bird from 1.6 km away, before their attempt to capture it (Smith 1997). Also in winter, individuals have been observed to hover, apparently using hearing before dropping through 20 cm of snow to catch voles (Smith 1997). When hunting from the ground, often use mounds, rocks, hills, but readily accept man-made structures such as fence posts, telephone poles, and Osprey nesting platforms on marshes.
In breeding season studies from Barrow, AK, Snowy Owls used two main methods to hunt: 1) perch and scan, and 2) hover and scan. Once prey is located, at least 5 different hunting techniques are used; chase, drop, glide, run, or slip. Capture was successful with two major methods; wallop or sweep. A wallop was defined as landing on prey on the ground, and sweep was defined as a fly-by and grabbing (sweep) prey with one foot while gliding in flight. Will hover-hunt if wind speed is strong enough to sustain the lift of its large body mass. Hover-hunting previously has been reported on the breeding grounds (Watson 1957).
One observer inferred a Snowy Owl was waiting for Eskimo hunters to flock-shoot flying King Eider (Somateria spectabilis) ducks along a lead in the ice, and pick-up the wounded ducks (Wiggins 1953). Same author reported another Snowy Owl to patrol a 1600 m length of radio-wire antenna and kill injured, or pick-up dead, Red Phalaropes (Phalaropus fulicarius), almost daily (Wiggins 1953). In a separate observation, a Snowy Owl learned to prey on injured ducks shot by native hunters and floating in water (see Robertson and Gilchrist 2003).
Apparently also opportunistic, and known to kill and eat Short-eared Owls caught in steel leg-hold traps in Alaska (Brooks 1915, Bailey 1948). Similarly, known to follow migrating Willow Ptarmigan (Lagopus lagopus) in the Russian Arctic (Alerstam 1982, p. 67), and concentrate at an artificial breeding facility of Arctic Hare (Lepus timidus) (Nagell and Frylund 1965). On the British Isle of Scilly, a wintering Snowy Owl was known to kill rabbits (Oryctolagus cuniculus) caught in snares, and feed on dead ones placed near the owls' roost (King et al. 1966).
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, and larger ones carried off. Reported to catch fish in Iceland, in same manner as Osprey do (Mikkola 1983). Fish have been infrequently reported in the Snowy Owl diet and appear to be of low importance (Bent 1938, Detienne et al. 2008, DWH).
During winter in s. Alberta, Snowy Owls used a sit-and-wait hunting technique in 50 of 51 attempts, and hovered once. Overall success rate was 43%, with adult females significantly more successful than juvenile females (Boxall and Lein 1982a). Juveniles appeared inept at handling some prey, as they were seen to drop prey twice, and or let prey escape after capture (Boxall and Lein 1982a). More observations on foraging times are needed for both breeding and nonbreeding periods.
In Sweden (Lind 1993), males hunted from higher perches than females; both rarely hovered. In the same study, older owls caught significantly smaller prey than younger owls. Adult females had a higher success rate at capturing prey than younger females did, but sample sizes were small.
Will chase small shorebirds and passerines during the breeding season and capture them in flight (DWH). Also known to pursue and capture large birds in pursuit flight such as: Canada Goose (Branta canadensis), Great Blue Heron (Ardea herodias), Northern Harrier (Circus cyaneus), and Peregrine Falcon (Falco peregrinus) (N. Smith, pers. comm.). Uses similar methods to steal (kleptoparasitize) prey from other birds, such as: gulls, jaegers, hawks, falcons, and owls (DWH, N. Smith, pers. comm.). Similar results observed during the breeding season (DWH).
Typical hunting sequence from studies at Barrow, AK, as follows: 1) perched owl in relaxed posture scanning the landscape; 2) upon seeing prey of interest, owl shifts to an erect posture, elongating body and extending neck – apparently for a better look; 3) pursuit follows with steady fast wing beats if on low perch, glide and few wing beats if on high perch. If hover hunting, owl slips downward with wings extended, occasionally flapping to brake during descent. The most common hunting technique was the pursuit, yet the owls did glide long distances from high perches. Will occasionally chase prey on the ground (See Fig. 2-6).
During this study, 36 non-breeding Snowy Owls were observed for 2 h periods, totaling 102 h. The owls were not individually marked, so some could have been observed more than once. Nonetheless, 24 of 36 owls were recorded to hunt 34 times, while 12 owls did not initiate a hunt. Of these 34 hunting observations, the owls used elevated perch hunting 30 (88%) times, flat ground perch hunting 2 (6%) times, and hovering 2 (6%) times. From elevated perches, the owls hunted from tundra mounds 17 (56%) times, telephone poles 11 (37%) times, man-made objects 2 (6%). They hunted only 2 (6%) times from level ground perches, and 2 (6%) times by hovering (DWH, unpubl. data).
Of these 34 hunting observations, 13 (38%) were successful, 12 (34%) missed, 5 (15%) aborted, 4 (12%) outcome undetermined. Methods to capture prey were: wallop, where owl lands on prey and grabs with talons (n = 10 successful, 6 unsuccessful), or sweep, where owl continues flight and grabs prey with one foot (n = 3 successful, 6 unsuccessful), (DWH, unpubl. data). See Fig. 7-8.
Depending on prey species, and size, feeding technique varies. Small prey such as lemmings, voles, and small birds are most often swallowed head-first (Watson 1957, Parmelee 1992, Detienne et al. 2008, DWH, pers. obs.). At other times, prey is dismembered and eaten in pieces, often starting with the head (Parmelee 1992).
Larger prey is pulled apart. The head is often dismembered, but sometimes not. On large prey such as ducks, the large muscles are eaten first (i.e. breast, humerus), and are usually the only areas eaten (DWH pers. obs.).
Major Food Items
During the Arctic breeding season, lemmings of various species are primary prey (Fisher 1893, Gabrielson and Lincoln 1959, Sutton and Parmelee 1956, Watson 1957, DWH, unpubl. data), or nearly so (Parmelee et al. 1967). Exceptions do occur (see below).
Snowy Owls are capable of feeding on whatever species are most abundant and easily captured in different habitats during any season. For example, in Hooper Bay, AK, Murie (1929) reported that food varied with rodents being taken almost exclusively in uplands, while waterfowl were taken to some extent in marshy areas. Also near Nome, AK, nesting pairs observed by Dufresne (1922) switched from rodents to ptarmigan when ptarmigan chicks hatched. On Fetlar I., Shetland where no lemmings occur, a nesting pair fed mostly on rabbits, shorebirds, and mice (Tulloch 1968, Robinson and Becker 1986).
In Barrow, AK, breeding adults supplement their diet with the onset of hatching waterfowl, shorebirds, and passerines (DWH). This switch is particularly important in years when lemming are scarce, for Snowy Owl chick survival (DWH). Individuals that are not breeding, or those that winter on islands free of mammals, may subsist on other prey such as birds.
Winter. On the wintering grounds individuals eat a wide diversity of prey, although most studies report mammals, with exception near coastal areas. Most studies report diet from pellet analysis, and few studies report > 100 prey items. Overall, wintering Snowy Owls eat a wider diversity of prey than breeding Snowy Owls do. Snowy Owls wintering in coastal habitats show greater dietary diversity than inland owls. See Table 1 for review of winter feeding ecology.
In coastal sw. British Columbia, birds comprised 100% of the Snowy Owl diet with grebes and ducks representing 80%. Of these, Horned Grebes (Podiceps auritus) and Bufflehead (Bucephala albeola) comprised 35% and 24% of total prey, and prey size ranged from 400–800 g (Campbell and MacColl 1978). The owls captured grebes off the water. In coastal New England, birds comprised 48% of the Snowy Owl diet as determined from 127 stomach contents from collected owls. Interestingly, 16 Dovekie (Alle alle) were recorded (Gross 1944). Based on 87 stomach samples from Maine, rats and mice comprised 35%, snowshoe hares (Lepus americanus) 20%, passerines 10% of the winter diet (Mendall 1944).
In s. Alberta, dissection of 100 pellets showed deer mice (Peromyscus maniculatus) the most abundant prey in 1976/1977 (61.2 %, n = 57) and 1977/1978 (51.0%, n = 79), followed by meadow voles (Microtus pennsylvanicus) (Boxall and Lein 1982a). Collectively, the two species comprised about 90% and 75% of the total prey (n = 240). Few other species were eaten, but included ground squirrels, weasels, jackrabbits, and partridge. Biomass calculations 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%. Difference in prey selection between the sexes was reported in one study, where male Snowy Owls preyed primarily on rodents, and female Snowy Owls on a wider and larger range of species (Boxall and Lein 1982a).
Unpublished data from N. Smith, at Massachusetts Logan International Airport study located in East Boston covers 32 yr. This study yielded > 6000 pellets, and numerous large bird and mammal carcasses. Norway rat (Rattus norvegicus) and meadow vole were the most dominant small to medium sized mammals identified. Small birds were also found.
Based upon nocturnal hunting observations, and retrieval of prey, Smith recorded larger prey that typically occurs only as fur or feathers in pellets. At least 30 identified species of large birds and mammals recorded in this way. Most common were Norway rat, Black Duck (Anas rubripes), while unusual prey included Canada Goose (Branta canadensis), Brant (B. bernicla), Herring Gull (Larus argentatus), Double-crested Cormorant (Phalacrocorax auritus), Great Blue Heron (Ardea herodias), Rough-legged Hawk (Buteo lagopus), American Kestrel (Falco sparverius), Peregrine Falcon (F. peregrinus), Barn Owl (Tyto alba = furcata), Snowy Owl, Barred Owl (Strix varia), Northern Saw-whet Owl (Aegolius acadicus), Short-eared Owl, house cat (Felis catus), mink (Mustela vision), and striped skunk (Mephitis mephitis) (Smith 1997, and pers. comm.).
Breeding. Few studies of breeding season diet in North America. On Banks I., Nunavut, Snowy Owls were known to kill adult and young Black Brant (Branta bernicla) (Manning et al. 1956). On Southampton I., Nunavut, Canada, Parker (1974) reported 97% lemming of 358 prey items. Of this, 96% were Dicrostonyx lemmings. Also the remains of 1 Arctic fox (scavenged?). On Agattu I., Aleutians island chain, AK, Ancient Murrelets (Synthliboramphus antiquus) composed 68% of the diet, other alcids 6.5%, and ducks 13.4%, based on pellet analysis (Williams and Frank 1979; also see Detienne et al. 2008 for review). Also known to favor storm petrels (Oceanodroma spp.) from Agattu I., AK, (L. Kenney, pers. comm.).
In Barrow, AK, diet recorded from pellets during breeding seasons from 1992-2013, reveal > 99% (n = 42,177) of prey were small mammals, with ~ 99% Brown and Collared lemmings (DWH, unpubl. data). Similarly, in same study, > 91% (n = 3092) of prey found cached at nests were small mammals, with ~ 99% Brown and Collared lemmings. However, cached prey at nests revealed more birds (~ 8%) in the owls' diet than pellet analysis would indicate (DWH, unpubl. data). Overall from this study, Snowy Owls ate 5 species of mammals, and 24 species of birds, including a few Snowy Owl nestlings (DWH, unpubl. data).
In detailed study of 100 breeding season pellets, there were significant differences among paired bones (i.e. jaws, humerus, pelvis, femurs, etc.) and skulls found in pellets (DWH, unpubl. data). Depending on the method used to determine diet from pellets, prey averaged 3 per pellet for mandibles and 2.7 per pellet for skulls (DWH, unpub. data). Because species can be identified using skulls, they are recommended. However, by quantifying prey found at the nest, species numbers and diversity increases as avian prey are more accurately tallied and identified. Data from prey cached at nests indicate the sex ratio of lemmings killed by the owls is about 1:1 (DWH, unpubl. data). Thus both methods should be used in describing breeding season diet.
To determine carbon and nitrogen stable isotope ratios for food-web studies, four captive Snowy Owls were fed lab mice for ~ 6 wk. Mouse muscle and owl blood was collected to determine δ13C and δ15N values. The diet-tissue discrimination factors (∆13C and ∆15N) for owl blood were +0.3% ±0.2%, and +1.9% ±0.1% (Therrien et al. 2011b). These results give promise that stable isotope research may provide further insight into Snowy Owl diet and feeding locations.
Food Selection and Storage
In most breeding studies, highly specialized on lemmings (Lemmus, Dicrostonyx) and occasionally voles (Microtus). On Fetlar I., British Isles, ate rabbits (Lepus) – the only abundant mammalian prey. In one study, captive owls selected mice over rats (Watson 1957). See Diet: Quantitative Analysis, above.
Female caches or stockpiles prey that the male brings to the nest. Caches of 10-15 individuals prey items are common. However, depending on lemming abundance and the hunting skill of the male, caches of 20-40 are not uncommon, and caches of 56 and 72 (DWH, unpubl data), and 83 (Pitelka 1955a) have been counted. From 1992-2013, 3,092 prey ere cached at or near nests in Barrow, AK (DWH).
Adults will also cache surplus prey away from nests. Males cache and retrieve prey to feed nesting females, nest-departed chicks, or themselves. Two caches away from nests were reported ~ 50 cached lemming each (Parker 1974). Caches may buffer food resources in times of prey decline, inclement hunting weather, or other factors. Or, the male may need to procure a certain number of lemmings in order to bring the female into breeding condition.
Sex of prey eaten is important for assessing predator-prey relations. At one nest in Barrow, AK, 83 lemmings were cached. Of these, 76 could be sexed; 25 females, 51 males. The average mass from 75 of these lemmings was 77.8 g males (n = 50), and 70.3 g females (n = 25) (Pitelka et al. 1955a). Also from Barrow, of Brown Lemmings cached at nest sites between 1992 and 2013, average mass was 74.6 g, n = 2,744 (DWH, unpubl. data).
Nutrition and Energetics
One captive Snowy Owl chick, 4 wk old, ate 3-9 (451.2 g) lemmings daily, going from 1079 g to 1676 g where it stabilized until its first flight. At this point it was eating about 20% (326 g) of its own body mass per/d. Its sex was not reported (Pitelka 1955a).
One juvenile male Snowy Owl in apparently good health was captured and kept in captivity for daily food consumption estimates over two trial periods (Seidensticker 1969). The owl weighed 1,292 g at capture. It was fed beef heart and mice and exercised minimally. During the 56 d winter (Jan and Feb) feeding period, the owl ate an average of 131 g, range 0-200 g/p/d, equaling about 8.6% of its 1,518 g body mass. It also showed no changes in body mass during the period. During the summer (Jul) 28 d feeding period, the owl ate an average of 145 g, range 0-298 g/p/d, equaling about 10.0% of its 1,445g body mass. It showed a 226 g change in body mass. Interestingly, during both trial periods the owl voluntarily fasted for 1-2 d at a time when food was available (Seidensticker 1969). The owl was captive and had no means of free flight or exercise and was offered unlimited food for 30 min/d. These results could produce some biases.
Other reports from laboratory studies estimate winter food intake as 284 g/p/d (n = 4 owls) (Gessaman 1972); 320 g/p/d (Mikkola 1983). Based upon energy budget studies, (Boxall and Lein 1989a) estimated 236-268 g/p/d. Based upon the number of prey per pellet, and the known mass of local prey, Detienne et al. (2008) estimated wintering Snowy Owls ate 150 g/p/d of voles.
Gessaman reported daily food intake for a captive owl were 200–400 g of lemming, at air temperatures between –4.5°C to –40°C, with the average gross energy intake equivalent to 400 g of lemming at –34°C. Once again these were captive bird results and wild active owls might require more than 400 g/p/d.
Average daylight (08:00-18:00) energy costs for Snowy Owls wintering in Alberta, Canada, reported at 511.2 kJ and 590.0 kJ for two seasons, respectively (Boxall and Lein 1989). When considering the 24 d period these numbers extrapolated to 1227.0 kJ/d and 1415.9 kJ/d, for the two seasons (Boxall and Lein 1989). These numbers then converted to average biomass/d of small mammals, yielding 236 g, 268 g, 273 g, and 309 g/d, depending on ambient temperature.
Biomass estimates from pellets are probably reliable for estimating the number of small prey swallowed whole. However, biomass estimates for large prey where only the breast and large muscles are eaten could give biased results. For example, Snowy Owls from Barrow, AK, eat some ducks, but only eat large breast muscles and muscles along the humerus and femur. Thus, if a 1,950 g King Eider is killed, and the owls only eat large muscles: these muscles may weigh: breast 312 g (16%), femur 110 g (5.6%), humerus 67 g (3.4%) or 489 g (25%) of the total mass (DWH, unpubl. data).
Snowy Owls are known to have thick (19-22 mm) subcutaneous fat during winter and believed to have the ability to survive 24-40 successive days without food (in Portenka 1972, in Voous 1988). This merits further study. For example, high Arctic populations of Willow Ptarmigan also store fat, but their reserves are thought to only last a few days (Stokkan 1992).
As a homeotherm, Snowy Owls maintain relatively constant body temperature. A captive Snowy Owl showed circadian variation in body temperature with lowest body temperature at night and highest during the day (Irving and Krog 1954, Irving 1955). Indeed, four observations from one Snowy Owl kept in an outdoor aviary with temperatures from -23°C to -11°C, registered a resting body temperature of 40.9°C (Irving and Krog 1954). See also Siegfried et al. 1975.
In another study from Barrow, AK, four female Snowy Owls were studied and later anesthetized to determine body temperature and heart rate (Gessaman 1978). Mean body temperature was 41°C (range 40.8° to 41.1°, for 19 d); 39.7° C (range 39.5° to 40°, for 7 d), 40° C (range 39.3° to 40.5°, for 5 d; and 41.1° C (range 40.8° to 41.4°, for 3 d). Average body temperature did not correlate with wind speed or ambient temperature in the outdoor aviary.
Considering body temperature results from Irving and Krog (1954) Siegfried et al. (1975) and his own studies, Gessaman (1978) concluded that individual Snowy Owls vary their body temperature by only about 1° C; among individuals, however, variation could be about 2.5° C.
Heart rate for three owls averaged 202 beats per minute (bpm) (range 189.6 to 222.3, for 7 d); 145.3 bpm (range 131.8 to 146.3, for 4 d); and 143.7 bpm (range 131.4 to 161, for 5 d) Gessaman 1978). As with average body temperature, heart rate did not correlate with wind speed or ambient temperature. During a 5 d starvation experiment, heart rate of one owl started at 202 bpm and dropped to 166.9 bpm (Gessaman 1978). The author believed these results reflected reduced level of daily activity and energy metabolism during the starving period (Gessaman 1978).
Four juvenile Snowy Owls were captured at Barrow, AK, and transferred to a laboratory in College, AK were tested for O2 consumption rates (Gessaman 1972). At ambient temperatures of 18.5 to 66⁰ C, with almost zero wind speed, the four owls showed an average lower critical temperature of 2.5⁰ C, and an extended thermal neutral zone of 18.5⁰ C.
The suggestion that daily body temperature fluctuations of Snowy Owls relates to their daily circadian rhythm needs more investigation. Observations during winter by many Snowy Owl researchers indicate the owls become more active as dusk. Furthermore, Snowy Owls are known to winter in the Arctic during these 24 h of darkness. Here they must hunt. Consequently, their daily body temperature should rise even in darkness. Therefore, similar physiological studies regarding body temperature, heart rate, oxygen consumption, and activity periods should be done on free ranging Snowy Owls living in 24 h of darkness during the Arctic winter.
Perhaps the Snowy Owl's large body mass, and presumably increased numbers and structure of feathers compared to other owl species, allow this owl to live throughout the year in the Arctic environment. The maximum insulation index (inverse of thermal conductance) of Snowy Owls is (91°C · cc O2 -1 · g-1 · h -1). Thus, Gessaman (1978) concluded the thermal conductance for the Snowy Owl was second to the Adelie Penguin (Pygoscelis acadicus) for avian species, and is equivalent to that of Arctic fox and Dall sheep (Ovis dalli). Whether these methods are relevant today needs to be reevaluated.
Drinking, Pellet-Casting, and Defecation
Captive birds drink water, especially after feeding (Watson 1957). Observed to stand in water and drink in wild and eat snow (DWH, pers. observ.). Not known if owls can substitute snow for water for prolonged periods.
Adults regurgitate pellets after stretching their head and neck upward. Unknown when chicks eject their first pellet but likely by about 10 d old. Little bone mass in chick pellets. Adult winter pellet, length x width from Washington averaged 80 x 31 mm (n = 62) (Patterson 2007), while in Montana mean length x width dimensions were 82 x 30 mm (n = 100) (Detienne et al. 2008). In Europe, length x width dimensions was 92 x 33 mm (n = 19) (Hagen 1960), and 66 x 26 mm (n = 108) (Cramp 1985). See Potapov and Sale (2012) for more pellet information from Russia.
Pellet mass reported to be 7.7 g from Michigan (Chamberlin 1980), and averaged 19 g from Montana (Detienne et al. 2008). In Europe, pellet mass reported at 13 g (n = 16) (Mikkola 1983). Prey per pellet in Montana averaged 5 (n = 100), and was similar to 4.3 reported in Europe (Hagen 1960).
Nest stays fairly clean throughout the egg-laying period, but becomes increasingly messy after young hatch owing to the accumulation of fecal waste, prey remains, pellets, and decaying carcasses.
Holt, D. W., M. D. Larson, N. Smith, D. L. Evans, and D. F. Parmelee (2015). Snowy Owl (Bubo scandiacus), version 2.0. In The Birds of North America (P. G. Rodewald, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bna.10