Many people wonder about Snowy Owl irruptions, and I thought it would be useful to copy the section on migration from the Birds of North America account on this species, written by David Parmelee in 1992, and send it to tweeters. Sorry that I can't include the graphics (Christmas Bird Count data graphed all across the continent to show the discordance among irruption years).

The way I see it, lemming abundance may influence the owls in two very different ways. First, lemming population crashes may cause the owls to seek new wintering areas, as their food supply is insufficient. But second, lemming population highs may produce very successful breeding, and the large number of offspring may then have to disperse farther to avoid competition with the adults and with each other (there are again too many of them for the current rodent populations). In the first case, the owl populations may be stressed, while in the second case they are doing very well.

Unfortunately, no one has been able to reject either of these hypotheses, and of course something else altogether might be involved, for example, weather patterns. I can see nothing in the data so far available that points to any effects of global climate disruption (aka global warming) on Snowy Owls. They are able to go wherever there is open country and microtine rodents.

Migration
Nature Of Migration In The Species
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.

Timing And Routes Of Migration
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.

Migratory Behavior
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.

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Dennis Paulson
1724 NE 98 St.
Seattle, WA 98115
206-528-1382
dennispaulson at comcast.net



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