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Snowy Owl

Bubo scandiacus

Order:
Strigiformes
Family:
Strigidae
Sections

Breeding

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Snowy Owl breeding habitat, Bathurst Island, Nunavut, June.

Snowy Owls favor wide-open expanses of tundra for breeding. During winter they are found in more varied habitats, but perhaps unsurprisingly they still favor open areas frequenting farmlands, beaches, and airports. They generally shun high elevation montane areas., Jun 20, 2008; photographer Gerrit Vyn

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Snowy Owl nest and eggs, Barrow, AK, June.

Snowy Owls nest on the ground in barren arctic tundra. , Jun 05, 2006; photographer Gerrit Vyn

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Snowy Owl chicks and eggs in nest, North Slope, AK,

Females brood nestlings, and simultaneously incubate unhatched eggs. Brooding continues, but intermittently, as chick’s age, and depart the nest in order of hatching – at about 3 wk of age. Smaller chicks, and eggs are ‘brooded’ by huddling under mass of larger chicks in nest.Image by Jenny Cunningham.

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Adult female Snowy Owl brooding chicks on the nest.

Several collared lemmings, previously delivered by the male lay nearby. Bathurst Island, Nunavut, Canada. June., Jun 25, 2008; photographer Gerrit Vyn

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Adult female Snowy Owl in flight, Bathurst Island, NV, June.

Female Snowy Owl returning to nest in flight. The brood patch, an area of bare skin used to warm eggs and chicks, is clearly visible.Image by Gerrit Vyn Photography.

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Adult male Snowy Owl delivering a collared lemming to a female on the nest.

The female is feeding a chick. Bathurst Island, Nunavut, Canada. June., Jun 25, 2008; photographer Gerrit Vyn

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Juvenile Snowy Owl, North Slope, AK,

Juvenal Plumage present primarily Jun–Aug, during post-natal dispersal on and near natal territory. Upperparts, greater, lesser and median wing coverts, and underparts downy and dark mouse brown, feathers tipped and speckled grayish white, those of scapulars and mantle less filamentous and faintly barred whitish; facial disk and chin white, the feathers tipped brown.Image by Jenny Cunningham.

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Figure 2. Schedule of breeding, migration, and molt for North American Snowy Owls.

Thick lines equal peak activity, thin lines off peak activity.

Phenology

Pair Formation

Pair formation and courtship behavior rarely observed, but there are good details from Bathurst I., Canada (Taylor 1973). Pairs tend to form on breeding grounds (at nests), once birds arrive back. Given migratory and nomadic habits, it seems unlikely that pairs form on wintering grounds (but see Behavior: sexual). Although some birds are known to winter on Arctic breeding grounds, most individuals arrive during Apr and May, with variation expected with latitude (Murie 1929, Pitelka 1955b, Fuller et al. 2003, Therrien et al. 2011a). Field observations from Barrow, AK, suggest females may be the more mobile sex, with adult males sometimes remaining on territory but not breeding (DWH). Courtship hooting is heard by mid-May in Alaska (Sutton 1932, Pitelka 1955b).

Egg Laying

In most studies, eggs are usually laid in mid-May to early June, depending on weather and food resources.

Nest Site

Selection

Options limited and dictated by snow-free areas. Male believed to establish and defend the territory, female believed to choose the specific nest site. More study needed.

Microhabitat

Within the tundra ecosystem, nests are located on dry areas with high relief, such as ridges, elevated mounds, high polygons, hummocks, hills, man-made mounds, sometimes marshy areas with high mounds, and occasionally rocky out-crops covered with vegetation (Holt et al. 2009). Most sites are windswept and receive less snow than lower relief areas. Thus, these sites melt snow first in spring and are the first exposed areas of tundra. Nesting sites often near the coast. Further details in Cramp (1985).

Site Characteristics

In Barrow, AK, most nests are on high-centered polygonal mounds about 1 m tall (range 95-106 cm); significantly different from random mounds (Holt et al. 2009). These mounds are created by the freeze-thaw cycle. Vegetation on mounds is low and does not conceal owls. Elevation reported from approximately sea level to 200 m on Baffin I., Canada (Watson 1957), 300 m Hooper Bay, AK, (Murie 1929), 30 m Barrow, AK (DWH).

Nest

Construction

Nest built and maintained solely by the female: digs soil with claws and bill, rotating and expanding the depression until a relatively circular shallow nest bowl is formed. Females may dig out a few nest bowls (“false beginnings”) before choosing one (Murie 1929). Fresh abandoned bowls rarely found in Barrow, AK (DWH). Needs more investigation.

Generally no nest material added, although molted body contour feathers are often scattered on the nest mound, and sometimes in the nest bowl. However, mosses, lichens, and grasses were reported in few nests from Hooper Bay, AK (Murie 1929); on Fetlar I., UK; grass was reportedly added to a scrape on a rock slope (Tulloch 1968). Similarly, Watson (1957) mentioned mosses and grasses sometimes found in nests.

No nest material reported for 239 nests from Barrow, AK (DWH). Most sites have commanding views of surrounding tundra (Murie 1929, Bent 1938, Pitelka 1955b, Watson 1957, Tulloch 1968, Parmelee 1992, Holt et al. 1999, Holt et al. 2009).

Two interesting observations: two different females dug alternate nests below and on the side of the original nest mound, when high winds and stormy weather created an unfavorable environment for the chicks. In one case, the female called the chicks to a lower nest where she continued brooding until the storm passed - then moved the family back to the original nest on top of the mound. In case two, the female kept the chicks in an alternate nest for several days as hard, cold, winds persisted (DWH).

Dimensions

In Barrow, AK, nest bowl length and width averaged, 47.7 x 44.0 cm (n = 91), and depth 9.6 cm (n = 68) (Holt et al. 2009). Murie (1929) reported 25-33 cm diameter and 4-9 cm depth. This is also consistent with reports from Norway (Hagen 1960, Portenko 1972, in Mikkola 1983).

Microclimate

No quantitative data. Nest sites generally on higher areas that receive less snow, and melt earlier than surrounding tundra. Sites usually receive almost continuous wind. Observations suggest females are comfortable in all but the most extreme cold summer rain or snowstorms. In fact, high relief of mounds may benefit owls from cooling winds during warm summer months, and relief from biting insects, such as mosquitos (Holt et al. 2009).

Maintenance Or Re-Use Of Nests

On Fetlar I., UK, Robinson and Becker (1986) imply that a “main nest” was used for 9 consecutive years. In Barrow, AK, only 1 of 239 nests reused in ensuing years (DWH). Overall, new mound used in almost all years. Nest maintenance not known, but female expands nest bowl to accommodate growing nestlings.

Nonbreeding Or Unfinished Nests

More data needed. Parmelee et al. (1967) observed a female construct a scape over a 3 d period, and then abandon it. Watson (1957) found many old scrapes from previous years, but only 1 new scrape that was not used. Rarely were new unused scrapes found in Barrow, AK (DWH).

Eggs

Shape

Elliptical to subelliptical, and glossy white (Baicich and Harrison 2005, DWH).

Size

Mean length x width for 56 eggs: 56.4 x 44.8 mm (Bent 1938). Mean length x width 57.4 x 45.2 mm reported in Baicich and Harrison (2005). Given highly nomadic lifestyle, unlikely that geographic variation in egg size exists for North America. From Europe, length x width averaged 57 x 45, (range 55-63 and 42-48, n = 100) (see Cramp 1985).

Mass

No precise data on newly laid eggs. From Canada, eggs weighed in various stages of development were: 52 g (n = 16), 58 g (n = 7) (Watson 1957). An 8-egg clutch had embryos that ranged in size from small to large; weights of the eggs ranged from 58.1 to 64.5 g (mean 62.1). From Europe, average mass 60.3 g (range 47.5-68, n = 66), and Norway 53 g, (range 50-59, n = 15) (see Cramp 1985).

Clutch mass reported to be equivalent of 25–43% of the female's body mass (Voous 1988). This is a wide range and will depend on clutch size.

Eggshell Thickness

No data.

Color

Fresh laid eggs white or creamy white (Bent 1938, Baicich and Harrison 2005) becoming stained from soil and debris in nest with age (DWH).

Surface Texture

Descriptions open to interpretation. Eggs reported to be smooth to somewhat granular with and without glossy appearance (Bendire 1892, Bent 1938, Baicich and Harrison 2005, Parmelee 1992). In Barrow, AK, all eggs generally smooth, somewhat glossy when fresh (DWH).

Egg Laying

The time between completion of a nest scrape and first egg laid is not known. Eggs thought to be laid any time of day during the Arctic summer's continuous daylight, but detailed data are lacking. In most studies, eggs are usually laid in mid-May to early June, depending on weather and food resources.

Egg laying intervals reported at 2 d, but 4 or 5 d also known. Intervals range from 41 to 50 h (Schaanning 1907, Pleske 1928, also in Bent 1938, Robinson and Becker 1986), or occasionally longer (Parmelee et al. 1967, DWH).

Robinson and Becker (1986) data well recorded. At one nest, 4 d elapsed between laying of the eighth and ninth eggs (but not between the ninth and tenth eggs), with a 4 d interval between hatching of the eighth and ninth chicks. Tulloch (1968) noted similar sequences, including a 5 d interval between laying, possibly related to inclement weather. Needs further study.

Replacement clutches suspected, but not confirmed. Replacement clutches never observed from Barrow, AK, but if egg failure happens early in the breeding cycle, re-laying seems likely. Needs study. Late clutches thought to be associated with bigamous mating (Watson 1957, DWH) or young inexperienced breeders (Robinson and Becker 1986, DWH), or lack of adequate food resources (DWH), not replacement clutches. Intraspecific egg dumping not reported.

Incubation

Onset

Only female Snowy Owls develop a brood patch, just prior to laying their first egg. Only the female incubates, beginning with the first egg.

Incubation Patch

As in all owls, female develops a single large incubation patch. Increases in reproductive hormones stimulate feathers from the lower breast, and abdominal region to fall out. Increase in blood flow results in highly vascularized and noticeable incubation patch.

Incubation Period

One of first North American reports calculated the incubation period at about 32 d (Murie 1929). More detailed records recorded a mean incubation period for the first egg of 31.6 d (n = 9), and close to 32 d for later eggs in clutch (Robinson and Becker 1986). Watson (1957) and Taylor (1973) reported 32 to 33 d. Incubation for the fourth, seventh and eighth eggs at one nest, and ninth and tenth eggs at another nest, 32 to 33 d (Parmelee et al. 1967). Reports of 37 to 38 d (Sutton 1932) not confirmed. Record of 27 d (Schaanning 1907) probably incorrect.

In one 10-egg clutch, hatching of last egg 15 d and 9 h later than the first egg (Pleske 1928, in Bent 1938). Of this 10-egg clutch, 7 eggs hatched out at 41 h intervals (Pleske, also in Bent 1938). Safe to say, incubation period about 32-33 d. If relevant, current ornithological definition of the incubation period is calculated from the last egg laid.

Snowy Owl females accomplish many feats during incubation. They must first have the endogenous reserves to develop and lay eggs, develop an incubation patch through loss of feathers, incubate eggs at a relatively constant temperature, brood nestlings initially unable to maintain their own body temperature, fast during lean hunting times, and begin the process of molt. How females meet the physiological and nutritional requirements to simultaneously accomplish the above behaviors is unknown and merits study. Do they lose body mass? Is thermal insulation lost? Do they use endogenous reserves?

Hardiness Of Eggs And Effects Of Egg Neglect

No quantitative data on hardiness of eggs. Female tend to be strict incubators, so eggs are rarely subject to freezing temperatures, rain, or wind chill. However, eggs unattended will freeze and crack open (Parmelee 1972). Drifting snow caused one female to abandon four eggs; whereas no desertions occurred at nearby nests that were essentially snow-free, emphasizing the importance of windswept sites (Taylor 1973). Overall, eggs are adapted to Arctic environments and females rarely leave nests unattended. Observations suggest females know when weather conditions are dangerous to eggs and chicks, and thus are more reluctant to flush from nests, and return more quickly, in harsh weather (DWH).

Hatching

Preliminary Events And Vocalizations

Chicks vocalize from within the eggs before hatching.

Shell Breaking And Emergence

Eggs usually hatched asynchronously, in order laid, and from the side of the large end of the egg; pipped eggs (hole/crack in shell) usually hatch in 1-2 d. Chicks appear to hatch at any time of day (DFP), but more data are needed (DWH). Hatching intervals range from 1 - 2 d (Muie 1929), about 37-45 h (Watson 1957, Tulloch 1968), 1-2 d (DWH), and 3 d (Parmelee 1992). Sometimes however, several days separate egg hatching (DWH). See also Incubation Period.

Female is not known to assist in hatching. Shells frequently remain in the nest, sometimes even slipping over unhatched eggs creating a double shell. This does not appear to affect hatching. Females are not known to remove eggshells from the nest; instead, shells are usually trampled, broken, or flipped from the nest mound (DWH). Sometimes eggs disappear from the nest with no indication of what happened (Robinson and Becker 1986, DWH).

Young Birds

Condition At Hatching

Chicks are semi-altricial. At hatching, chicks are wet, covered in white protopile (1st) down, with eye-lids fused shut, and white egg tooth present. Bill is black and cere grayish or bluish. Large metatarsal pad and soles of feet are bare of feathers, and pink in color. By end of day one, chicks dry into a white fluffy downy plumage. They are helpless, move little, and dependent on the female for food, thermoregulation, and protection. A soft twitter-only vocalization is heard at this time, believed to be associated with hunger or discomfort (DWH).

Similar descriptions of emerging chicks reported from Hooper Bay, AK; Southampton I, Nunavut; Norway; and Fetlar I., UK -- where chicks hatch-out in white down, skin pinkish, cere pale blue, bill bluish, eyelids bluish, irises blue-white to pale-grayish, becoming yellower with age (Murie 1929, Sutton 1932, Hagen 1960, Tulloch 1969, Potapov and Sale 2012). Smaller chicks crawl under larger chicks apparently for warmth, but move little at this age (DWH).

Body Mass At Hatching

On Baffin I., Nunavut, 7 newly hatched chicks ranged from 35 to 55 g (mean 46 g; Watson 1957), while mean weight of 3 chicks from Victoria I., Canada, were 44.7 g (Parmelee et al. 1967). In Norway, newly hatched chicks weighed 42-45 g (Hagen 1960). Some newly hatched chicks may lose up to 45% body mass, while others gain mass daily (Watson 1957). Further study of this desirable.

Growth And Development Within The Nest

Data on growth and plumage development exists, but sample sizes are small and irregular recording of mass gain has made interpreting most of these studies difficult (Murie 1929, Pitelka 1955a, Sutton and Parmelee 1956, Watson 1957, Hagen 1960, Tulloch 1968, 1969, Parmelee 1972, Taylor 1973, Robinson and Becker 1986, Potapov and Sale 2012).

Nonetheless, three studies provided adequate mass gain and plumage development information (Watson 1957, from Canada; Hagen 1960, from Norway; and Tulloch 1968, from Great Britain). The most complete data set on mass gain and plumage development comes from DWH's study at Barrow, AK, where data derived from more than 162 nestlings (DWH); also Table 2 and Table 3 .

No overt aggression observed, nor evidence of sibling cannibalism by older chicks killing younger chicks (some squabbling occurs; DWH). Tulloch (1968) and Sutton and Parmelee (1956) never observed females kill young chicks to feed to other nestlings, nor observed larger nestlings kill smaller ones. Females fed all chicks, although older ones were usually fed first. Nest observations from Barrow, AK, indicate female's apportioned prey to all chicks (DWH). No sibling cannibalism observed, but female did feed dead nestlings to members of her brood or herself.

Nestlings depart nests on foot, flightless at about 3 wk of age (Romero et al. 2006, DWH).

Pre-Fledging Nest Departure Period

We define the time between leaving the nest on foot, and first sustained flight as the nest departure period (DWH). Oldest chicks leave first, followed by younger chicks as they approach 3 wk of age. Before that time, chicks can be out of nest bowl, but still remain on the nest site (i.e. the nest mound), 1-2 m from the bowl. Thus, definitions of leaving the nest explain the wide range (14-26 d) of nest departure dates reported (Sutton and Parmelee 1956, Watson 1957; Pitelka et al. 1955a).

After departure, chicks may occasionally wander back to the vicinity of the nest (Parmelee et al. 1967, Robinson and Becker 1986, Watson 1957). Chicks still depend on parents for food and protection from predators, but must withstand ambient environmental conditions. Although chicks are located individually, they tend to stay near each other and at times are found roosting together (Parmelee 1972, DWH). Chicks from Barrow, AK, departed the nest on foot at about 22 d (Romero et al. 2006, DWH).

In Barrow, AK, nest departure dates were not correlated with brood size, feeding rates, or sibling competition (Romero et al. 2006). Instead, nest departure probably evolved as anti-predation strategy, leaving the nest before it located by predators (Holt and Leasure 1993). After nest departure, chicks remain mobile. They walk well (bi-pedal) and may stay near the nest (Hagen 1960) or wander up to 1 km or more; this behavior is dependent on prey availability, and density of nesting owls (DWH). Most chicks range within 500 m of the nest but other authors have reported distances of 250 m, 400 m, and 1 km (Watson 1957, Tulloch 1968). Most hide alone, but near each other in tundra nooks and crannies. Occasionally, chicks are found in sibling groups. Brood mixing did not occur in Barrow, AK (DWH), or for bigamous nesting pairs on Baffin I., Canada (Watson 1957).

Adults usually remain in view of their chicks, with females roosting closer to chicks than males do. Attempts at flights reported from 30 d onward, but sustained flights, or fledging, occur about at about 50 to 60 d of age (Hagen 1960, Watson 1957), 45-50 d in the UK and Alaska (Robinson and Becker 1986, DWH).

Behavior

At young age, nestlings react to handling by grabbing weakly with feet and struggling with wings (DWH). Threat postures are first noticeable 20 to 25 d after hatching and commonly after 28 d (Watson 1957), but may occur sooner (DWH). Whatever their age, at final departure, they are quick and agile afoot.

Parental Care

Brooding

Only females brood nestlings, and simultaneously incubate unhatched eggs. Brooding continues, but intermittently, as chick's age, and depart the nest in order of hatching – at about 3 wk of age. Smaller chicks ‘brooded' by huddling under mass of larger chicks in nest. In extreme weather, chicks will huddle under females' wings (DWH). Males not known to brood. Female usually remains in the nest for food deliveries but sometimes flies to male to receive food; also leaves nest to defecate and for stretching, feeding. Young that leave the nest are not brooded in the field, and deal with weather on their own.

Feeding

Male brings food to the incubating/brooding female, who in turn feeds young. Female tears apart prey and feeds chicks small soft portions. Young are given increasingly larger portions up to the time they receive whole prey (Watson 1957, Parmelee 1992, DWH). Male generally does not feed chicks on the nest, and only delivers whole prey to the young once they depart the nest (DWH). Female leans forward and sways head as male approaches with food. Sometimes females fly from the nest to take prey from the male. Females may hunt in nest vicinity, especially as chicks get older.

No data on feeding rates of male to female, or male to female and nestlings. No quantitative data on how food is apportioned among chicks. Some die in nest in all years, including years of high prey abundance. Serious sibling aggression and cannibalism not observed from Barrow, AK (DWH). Successful departure from a nest by all or most siblings suggested that asynchronous hatching and asynchronous growth rates matter little when sufficient food is available. Dispersed young receive prey from adults and generally swallow prey (e.g., lemmings and small birds) whole.

Nest Sanitation

Nest stays relatively clean during incubation. Females move off the nest to defecate. However when chicks hatch and defecate, and prey remains pile up, nest becomes messy with eggshell remains, waste products, decaying animal carcasses, and pellets. This attracts flies that lay eggs on decaying carcasses (Parmelee 1992, DWH). Flies and other invertebrates occur at nests, but not known how these species interact with Snowy Owl chicks and adults. A detailed study on the invertebrate community at a Snowy Owl nests is desirable.

Immature Stage

More data needed. Initially, thought to be independent of adults by late Aug, or early Sep (Pitelka 1955b). More likely chicks are not independent until the onset of migration, in late Sep, or early Oct (DWH). Data from Britain and Barrow, AK, indicate independence by Oct (Tulloch 1968, DWH). Chicks thought to begin to hunting about 5 wk after leaving the nest (Watson 1957). More data needed.

Recommended Citation

Holt, Denver W., Matt D. Larson, Norman Smith, Dave L. Evans and David F. Parmalee.(2015).Snowy Owl (Bubo scandiacus), The Birds of North America (P. G. Rodewald, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America: https://birdsna.org/Species-Account/bna/species/snoowl1

DOI: 10.2173/bna.10