Breeding

Pair Formation

Little information. On the breeding ground, presumably not before late Apr to mid May (Voous 1989; Fig. 2), but pairs may actually form on the wintering grounds in the northern Great Plains. Incipient courtship activities as early as midwinter in s. Alberta (Boxall and Lein 1982c). Whether pairs actually bond there and arrive together on the breeding ground is debatable.

Selection Process

Presumably the male establishes the territory and the female initially selects the nest site, but this subject merits further study.

Microhabitat

At the onset of laying, the female visits one of several possible sites, usually on some windswept prominence, not necessarily the highest in the vicinity. In high, rolling country such hillocks are numerous and widespread. In flat, marshy areas they may be decidedly scarce, but nevertheless essential, if only amounting to a few hummocks or frost-heaved polygons. Nests on top of large boulders are exceptional in some areas (Murie 1929) but not in others (Watson 1957).

Site Characteristics

In addition to a sufficient food supply, the site must be snow-free, not subject to flooding, and command a view of its surroundings.

Construction Process

Nest constructed solely by female, so far as known. On turf or bare ground, female scratches out a scrape with her claws. Here she moves about and even twirls, until a distinct but shallow hollow results. No insulating lining of feathers or vegetation is added, though such material may accidentally fall into the depression.

Dimensions

No information.

Microclimate

Nests often placed in windswept places where snow and moisture tend not to accumulate. Cold does not appear to be a factor as long as the female covers eggs or small young. Eggs and small chicks are totally dependent on insulative protection of the female’s incubation patch, as an unlined scrape has no insulative value.

Maintenance Or Re-Use Of Nests

Scrapes may be used repeatedly over long periods (Watson 1957). At an unusual breeding ground in the Shetland Islands where owls bred for nine consecutive years, Robinson and Becker (1986) imply that a “main nest” was used repeatedly.

Nonbreeding Or Unfinished Nests

For reasons unknown, the female may work at one scrape and then abandon it for another nearby. Parmelee et al. (1967) observed a female that worked over a scrape during a three-day period before abandoning it.

Shape

Short elliptical or subelliptical (Harrison 1984).

Size

Measurements of 56 eggs show four extremes: 60.5 x 47 mm, 58 x 47.5, and 50.6 x 4l.7; mean measurements for all were 56.4 x 44.8 mm (Bent 1938). Mean measurements given by Harrison (1984) are 57.4 x 45.2 mm. No geographic differences in size across the Northern Hemisphere; 20% smaller than those of northern Eagle Owls (Bubo bubo) and 8% smaller than those of northern Great Horned Owls (Voous 1989).

Mass

Not well known (for fresh eggs), in part because incubation begins with the first egg. In a clutch of seven eggs, each weighed a mean of 58 g (Watson 1957). An eight-egg clutch had embryos that ranged in size from small to large (feathers); weights of the eggs ranged from 58.1 to 64.5 g (mean 62.1; DFP). Weights given by Y. Hagen (in Watson 1957) range from 60 to 65 g and probably approximate the weight of fresh eggs, but this subject merits further study.

Clutch mass equivalent to 25–43% of the female’s body mass—a significant physiological achievement and exceptional for large owls and large raptors in general (Voous 1989).

Eggshell Thickness

No data.

Color

White or creamy white (Bent 1938, Harrison 1984). As incubation proceeds, the whitish shells often become stained.

Surface Texture

Smooth to somewhat granular. A few “corrugated lines” are noticeable in some specimens (Bendire 1892, DFP). Some eggs lack gloss (Bent 1938) but others show a slight gloss (Harrison 1984).

Egg Laying

Interval between completion of a nest scrape and laying of first egg not reported, nor has the precise time of egg laying. Eggs apparently laid almost any time during the 24-h period of continuous daylight.

Eggs usually laid at intervals of about 2 d. Mean intervals range from 41 to 50 h (Schaanning 1907; Robinson and Becker 1986), occasionally longer (Parmelee et al. 1967). At one nest, 4 d elapsed between laying of the eighth and ninth eggs (but not between the ninth and tenth eggs), with consequent 4-d interval between hatching of the eighth and ninth chicks. Tulloch (1968) noted similar disruptions, including a 5 d interval between layings, possibly related to inclement weather. This subject merits further study.

Egg attentiveness by the female commences immediately with the laying of the first egg. Throughout the egg-laying period, the male guards the site. Replacement clutches suspected but not confirmed. Small clutches appearing late in the season suggest replacements, but late clutches also relate to bigamous matings (Watson 1957), possibly also to young birds (Robinson and Becker 1986). Intraspecific egg dumping not observed.

Onset Of Incubation

Female alone incubates, commencing with the first egg. No exception reported.

Incubation Patch

Female develops a conspicuous incubation patch—an enormous, flabby, highly vascularized featherless area of pink belly skin used in covering the eggs (Parmelee 1972).

Incubation Period

Mean period for the first egg over a 9-year period reported as 31.6 days, slightly longer (32 d) for later eggs (Robinson and Becker 1986). Watson (1957) and Taylor (1973) reported 32 to 33 d. The period for the fourth, seventh and eighth eggs at one nest, and for the ninth and tenth eggs at another, reported as 32 to 33 d (Parmelee et al. 1967). Extreme records of 27 d (Schaanning 1907) and 37 to 38 d (Sutton 1932) not confirmed.

Parental Behavior

The duration of the female’s attentive period probably varies individually. Toward the end of incubation, females become more intermittent in their incubation (Taylor 1973). On the other hand, one female noted by Parmelee et al. (1967) remained at the nest as long as eggs (or young) were present; another continued to incubate an addled egg (no embryo) as late as 1 Aug after the young had left.

Hardiness Of Eggs And Effects Of Egg Neglect

Eggs are subject to below-freezing temperatures, particularly in May. When left unattended during severe weather, they will freeze hard 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). Neglect of certain eggs results in mortality, likely more through accident than design. One female, brooding chicks during inclement weather, failed to cover an egg that should have hatched on 18 July; although the chick within the shell was still alive on the 21st, it died before hatching (Parmelee et al. 1967).

Preliminary Events And Vocalizations

No information.

Shell Breaking And Emergence

Starring may occur either at the end or side of the shell. A small circle of fine cracks appears, followed by a gradually enlarging and outward lengthening of the cracks. Pipping occurs when the chick finally punches a hole through the weakened circle of cracks. Several hatchings observed during the afternoon hours, but presumably they occur at most anytime in the nearly continuous polar daylight (DFP). The period from the first cracking of the shell until the first small hole was at least a day in three cases; the period from first visible crack to the final emergence of the chick was well over a day but less than 2 d in three cases, and less than 1 d in another (Watson 1957). Periods up to 3 d have been observed (DFP), indicating that the period from starring to emergence may be variable.

Eggs generally hatch at 2 d intervals except when egg laying has been interrupted, resulting in longer periods between hatchings. Female apparently gives no assistance during hatching; no evidence that she carries away eggshells. Some shell remains are trampled into the ground within the nest scrape or close by. Captive females also eat shell remains (Watson 1957), and presumably wild birds do likewise.

Condition At Hatching

Semialtricial; newly hatched chicks leave the shell wet, blind, and seemingly helpless. Eyes are closed at hatching and start to open usually by the fifth day, occasionally later. Within a few hours the newly hatched chick is covered nearly completely with rather short, white, fluffy down that extends to the base of the bill and claws, but with a bare patch at tarsal joint. Cere pale blue, bill bluish, palest on the horny part of upper mandible, becoming darker with age; eyelids bluish, irises blue-white to pale-grayish, becoming yellower with age (Sutton 1932).

Weights (to nearest 5 g) of seven newly hatched chicks ranged from 35 to 55 g (mean 46 g; Watson 1957). Mean weight of three was 44.7 g (Parmelee et al. 1967). Linear measurements lacking for live chicks. One preserved specimen has an approximate body length of 100 mm, bill from cere 11.0 mm, middle toe claw from sheath 5.5 mm; another had a body length of 99 mm, bill 10.00 mm, claw 5.0 mm.

Growth And Development

In the most complete study to date, some chicks lose much weight the first day (as much as 45% before recovering), while others increase greatly from the start (Watson 1957). Some chicks increase their weight by 60%–70% in a day. Nearly all grow rapidly by the third day, but increase less thereafter, falling to about 20% by the eighth day, and to about 6% in the fourth week. Some 28-day-old young weigh 1,300–1,600 g.

Some 4-day-old chicks show noticeably enlarged tarsal pads, claws, and bill, which still retains an egg tooth (DFP). Chicks about 9 days old have prominent bills (16.5 mm from cere, no trace of egg tooth) and claws (14.0 mm from sheath of middle toe). First signs of incoming additional down feathers appear along all of the major feather tracts in some 4-day-old young (DFP). By as few as 8 d, or as late as 12 d, the short white down is largely replaced directly by long, fluffy, dusky colored down with grayish tips, giving the owlet a dark appearance touched here and there with frosty patches, particularly in the head and body regions. Incoming remiges are short and sheathed nearly to the tip in some chicks 10 d old. Feathers emerge from their sheaths in some chicks 11 and 12 d old (Watson 1957). Growth of the gray down and feathers varies among chicks (DFP).

Nestlings react to handling by bill snapping and hissing, and are easily immobilized (tonically immobile) by turning them over on their backs or simply picking them up (Watson 1957). In such a state the body and legs stiffen, and even the stretched wing remains motionless. The eyes in some are closed, but those of others remain open. Fairly large young handled by Parmelee (1972) were so stiff that they were easily balanced on a point of rock. The significance of this behavior is not well understood, but evidently it occurs in other owl species as well.

Threat postures are first noticeable 20 to 25 d after hatching and commonly after 28 d (Watson 1957). Little information is available on intersibling conflict, a condition less likely in this species than in other owls because the young scatter over the tundra long before fledging. Young move about actively within the nest scrape where small young often find shelter beneath their older siblings. Nestlings are capable of leaving the nest when only 14 d old, though most remain a few days longer. Whatever their age of final departure, they are quick and agile afoot at that time. Flight happens slowly, usually long after the nest has been abandoned. The owlets spring from the ground and beat their wings furiously, only to fall back many times before accomplishing even a short flight (Parmelee 1972).

Brooding

Female broods the young from hatching until they abandon the nest. The asynchronous hatching results in brooding young and incubating eggs at the same time. Neither brooding of young by the male nor brooding outside the nest by either sex has been reported. Unless the brooding female is disturbed, she remains at the nest without relief from the male except for feedings. Female alone broods. No exceptions reported.

Feeding

Newly hatched young may eat partially digested food taken from the adult (Collett 1921). For the first two days owlets are fed on lemming soft parts, including heart, liver and testes, which the female tears out and feeds in small pieces (Watson 1957). Some 5 d old young may be fed parts containing bones, and those 10 d old may be given partially dissected lemmings. Some week-old owlets already bring up long pellets of lemming fur and bones. Although young 14 d old are capable of handling their food, most feed themselves very little. The female may cast up pellets that are fed directly to the larger nestlings (Robinson and Becker 1986). Dispersed young handle undissected lemmings and other prey.

Male brings food to the incubating/brooding female, who in turn feeds the young. Young away from the nest are fed by the male, at least until the female abandons the nest. More information is needed to determine the role of the female following her activity at the nest. Female dissects prey into small portions before feeding small young. Older young are given larger portions up to the time they receive whole prey. Male feeds only whole prey to their young. More information is needed on the parent offspring break.

No data available on frequency of feeding trips, amounts of food brought per trip, and apportionment of food to young. The fact that siblings do not always gain body mass equally suggests unequal apportionments of food and possibly sibling aggression. On the other hand, the successful departure from a nest by all of its siblings indicates that uneven growth rates matter little when sufficient food is available.

Nest Sanitation

Female defecates away from the nest, which remains quite clean before the young begin to hatch. Thereafter, the nest becomes increasingly soiled through defecation (no fecal sacs) by the young, disgorged pellets, and particularly unused lemming carcasses and other prey that accumulate and spoil. Flies gather at nests that become soiled after hatching of young. Mosquitoes attack owlets, particularly about the eyes and face (Sutton and Parmelee 1956, Watson 1957). Not determined is whether such attacks are simply annoyances, or vectors of disease. A detailed study on the invertebrates at a Snowy Owl’s nest is needed.

Little information. Females and broods of bigamist matings remain apart and do not interact within the males’ territory (Watson 1957).

Departure From Nest

On Baffin Island, Parmelee and Sutton (1956) observed that owlets left the nest when 14 d old, although Watson (1957) found that the period ranged from 18 to 25 d; most young left about 25 d. On Victoria Island, Parmelee et al. (1967) found that most young left at 14 to 16 d, some remaining until 22 d of age. Having left the nest, young may (Parmelee et al. 1967, Robinson and Becker 1986) or may not (Watson 1957) return. Although some owlets leave the nest and wander when only 14 d old, some of them apparently return to the nest one or more times before abandoning it for the last time. A final departure date of 25 to 26 d seems reasonable; many days pass before even these late-departing young fledge. Under the care of both parents, the scattered young sometimes reconvene and band together (Parmelee 1972).

Fledglings make frequent short flights, especially down inclines, before they are capable of making sustained buoyant ones. Owlets are unable to fly strongly with good control until over 50 d old (Watson 1957, Parmelee et al. 1967, Robinson and Becker 1986). At one nest with 11 eggs, 54 d elapsed between the laying of the first egg and hatching of the last young; allowing another 50 d for fledging, the interval from first egg to fledging of the last young totaled 104 d (Parmelee 1972). The period would be considerably shorter for nests with small clutches.

Once the young leave the nest (at about 25 d old), they are fed entirely by the parents for at least another five weeks, when they first began to hunt for themselves (Watson 1957). They are probably partly fed for at least another week or two, as they are still poor at hunting. During the five weeks after leaving the nest, an owlet eats roughly 7 kg of lemmings. Including lemmings eaten in and out of the nest, a brood of nine owlets will have consumed as many as 1,500 lemmings by the time they reach independence.

Little information on independent, fledged young on the breeding grounds. See Migration; Populations: range.