Migration

Not predictable. Migratory movements relate in ways not fully understood to the abundance of prey species, thought by some researchers to vary considerably from region to region across the polar tundra.

The classic explanation of the species’ irruptions implied that the owl’s principal prey, lemmings, crashed along a broad tundra front following a four-year peak, forcing the owls to wander in search of prey. The fact that many Snowy Owls appear in s. Canada usually one winter in four suggests that these irruptions are linked to the lemming cycle, although admittedly the flights sometimes occur in two successive winters, the second involving smaller numbers (Fig. 3; Godfrey 1986).

Recent observations on wintering Snowy Owls challenge the implication of the classic hypothesis that a coordinated lemming-owl cycle is geographically synchronous continent-wide. Although these owls show unpredictable periodic irruptions in eastern and western North America, a large majority appear to be regular migrants over much of their winter range, notably in a central zone including the northern Great Plains where they are especially abundant (Figs. 1, 3; Kerlinger et al. 1985). Lacking hard evidence that lemmings fluctuate synchronously over vast areas (Maher 1970), Kerlinger and associates believed that the microtines occupied a mosaic of tundra patches varying in size. Owl dispersion even in the favored winter midlands is patchy, and the abundance of owls within patches may vary markedly between seasons. Dispersion is thus thought to be linked to local availability of their primary prey (Kerlinger and Lein 1988b).

Looking at the geographic dispersion of lemming abundance and Snowy Owl breeding during any given year yields a mosaic. Peak numbers of varying lemmings (Dicrostonyx groenlandicus) at high latitudes in North America appear to be far less than peak numbers recorded at lower latitudes where both the varying lemming and brown lemming (Lemmus trimucronatus) occur in the same areas, or in those areas where extraordinary numbers of only brown lemmings occur at times (Parmelee 1972). Also, lemming fluctuations appear to be less regular at the higher latitudes where owl clutches average smaller, judging by a long-term study on Bathurst Island at 76°N in the Canadian Arctic. Compounding the problem of synchronizing owl breeding with lemming peaks is the fact that at least a few owls may breed during periods when lemmings are only moderately abundant. Nevertheless, the crux of the phenomenon is a mobile breeding population of owls that move nomadically, breeding where and when their prey is abundant.

Not so easily explained are the large, geographically synchronous winter irruptions that are observed in eastern and western North America (Fig. 3). Breeding areas for the birds involved in these irruptions must necessarily be on the order of millions of hectares, far greater than can be attributed to lemming mosaics (Kerlinger et al. 1985). Other factors, possibly weather, i.e., the relationship between snowfall and temperature conditions on the breeding grounds, could be important. Herein lies a crucial, little known area in the species’ makeup that merits additional research.

Apparently do not wander aimlessly on the winter grounds, at least during regular migrations to the Great Plains. Many that winter in s. Alberta defend territories (Boxall and Lein 1982b), as they do in Wisconsin (Keith 1964); winter site fidelity demonstrated through banding of individuals (Oeming 1957, Follen and Leupke 1980).

Sex and age classes overlap little on winter range. On average, immature males winter farthest south, adult females farthest north, with adult males and immature females in between (Kerlinger and Lein 1986). This arrangement probably due to “social dominance;” observations suggest females dominant over males, mediated by territoriality. Primarily first-year birds (nonbreeders) occupied areas subject to irruptive movements east and west of the northern Great Plains.

No information.