Demography and Populations

Age At First Breeding

Not known. Inasmuch as Pitelka et al. (1955a) observed nonbreeding individuals thought to be immatures one year old, at least two, possibly more years are required for sexual development. Under certain, perhaps unusual conditions, some individuals breed annually when not dependent on lemmings. A male that was mated to two females bred for nine consecutive seasons until he disappeared (Robinson and Becker 1986). Considering the irregularity of lemming abundance, it is conceivable that some individuals breed only once in every three to five years.

Clutch

Highly variable, depending largely on the availability of food. The clutch size is generally small (3–5) when food is limited, but large (7–11) when plentiful. Clutches in excess of 11 eggs (as many as 16 reported) need confirmation. Nesting is aborted when food is extremely scarce. Normally one clutch per breeding season. Repeat nestings following early loss of eggs is suspected but not confirmed.

Annual And Lifetime Reproductive Success

Difficult to determine due to nomadic behavior of birds. Number of young produced per pair/nest/clutch may be highly variable among neighboring pairs. One nest produced 11 young from 11 eggs (at least 10 fledged) while another nearby produced only four young from 10 eggs (Parmelee et al. 1967). Under very favorable conditions, reproductive success can be extraordinary. Of 32 eggs observed by Watson (1957), 31 hatched and all 31 young fledged. This observation shows that an entire group of nesting females under surveillance can succeed. Females not known to produce more than one brood per season, the limiting factor perhaps being the extra-long breeding season.

Maximum known age in the wild is 9 yr, 5 mo (Glutz and Bauer 1980; DFP). A captive bird in Switzerland lived for at least 28 years (Schenker 1978), an age that may be attained in the wild. Inasmuch as the Snowy Owl is simultaneously nomadic, migratory and irruptive, the species may have reached a terminal point in its evolution, unless it is able to develop other means of avoiding winter, life-wasting migrations, and irruptions (Voous 1989).

Many Snowy Owls that move southward from arctic regions are mistakenly assumed to die from starvation. Although this may prove to be the case during irruptive migrations of young in western and eastern sections of North America, there is no evidence that this is so in the N. Great Plains. In Alberta, 45% of the specimens examined had moderate to heavy fat deposits, and traumatic injuries were the major cause of mortality (Kerlinger and Lein 1988a). Causes of death or injury were collisions with unknown objects (46.5%), automobiles (14.1%), utility lines (4.2%) and airplanes (1.4%); also gunshot wounds (12.7%), electrocution (5.6%), fishing tackle (1.4%). Only 14.1% was believed due to starvation. Even as far south as Kansas, a Snowy Owl fed on rodents at a lumberyard for nearly a month before being accidentally electrocuted (Parmelee 1972). Gross (1947) inferred that individuals seen far from land at sea never live to return, but this is a moot question, difficult to resolve.

Prolonged periods of inclement weather subject young to diseases. An owlet that died in the nest in the Shetland Islands was infected with pneumonia and contained Staphylococcus, and another harbored the fungal infection Aspergillosis, both conditions indicating dampness (Robinson and Becker 1986).

Natal Philopatry

Not well documented. A prefledged Snowy Owl banded by Parmelee et al. (1967) in 1960 was shot by a native hunter three years later within 70 km of its natal ground on Victoria Island in the Canadian Arctic. It was not determined whether this individual was breeding, although in this same area at least one pair raised young that season. Another prefledged owlet banded on Victoria Island in 1960 (Parmelee 1972) was recovered nine years later at Southampton Island in November far from the natal site; not determined was whether this individual had bred where recovered. Conceivably, nomadism would be selected over philopatry if the owls are truly nomadic in their breeding as appears to be the case.

Fidelity To Breeding Site And Winter Home Range

Not documented for breeding sites, but some evidence exists for winter site fidelity (Oeming 1957, Follen and Leupke 1980).

Dispersal From Breeding Site

Seven young siblings were banded by Parmelee (1972) on Victoria Island on 18 July 1960. One was recovered in e. Ontario near Clyde Forks on 19 October 1961, another near Hudson Bay at Attawapistat, Ontario on 8 May 1962, and a third, on the opposite side of the world, at Sakhalin, USSR, on 18 February 1962—all three recoveries within seven months of one another. This extraordinary dispersal of young also explains in part why the species is nomadic and monotypic.

Home Range

No information.

Numbers

Although populations have been estimated on the breeding grounds, most cover a relatively small area rather than an entire island or region. In perhaps the most meaningful census covering a broad area, Manning et al. (1956) estimated 15,000 to 20,000 owls in the Canadian Arctic on Banks Island (about 64,000 km2) during a reproductive high, but only 2,000 during a low. The number of individuals seen per hour of observing was multiplied by a figure related to the conspicuousness of the species to estimate the number per square mile. Considering the conspicuousness of the owls, aerial reconnaissance used for censusing waterfowl and other wildlife would be desirable.

Wintering owls recorded at 235 sites of Christmas Bird Counts, generally in low numbers (Root 1988). Highest abundance (0.21 individuals/party hour) recorded near Green Bay, WI, where the owls were seen in 80% of the years examined. Winter censuses are disrupted by periodic irruptions of owls, especially outside the northern Great Plains.

Seventeen irruptions between and inclusive of the years 1882 and 1883 and 1945 and 1946 took place in North America with intervals of 3 to 5 years (mean 3.9 yr). Numbers of owls were estimated in the thousands (Gross 1947). For reasons unknown, during such times the owls fly far from land over sea where they take refuge on ships. The distribution map based on Root’s (1988) analysis substantiates the hypothesis by Kerlinger, Lein, and others that the majority of Snowy Owls in North America winter regularly in the northern Great Plains.

Trends

Except for an apparent decline of populations in northern Europe (Voous 1989), information is lacking for long term shifts in the numbers or distribution of this species.