Irruptive migrations

Like the irruptive finches, some rodent-eating species respond to periodic crashes in their main food supply by winter emigration, appearing south of their breeding range in much larger numbers than usual. Irruptions of Snowy Owls Nyctea scandiaca from the tundra to the boreal and temperate regions of eastern North America have been documented at least since 1880. Throughout the next 120 years, irruptions occurred every 3-5 years, at a mean interval of 3.9 (SE ± 0.13) years (Newton 2002, Table 19.4). Moreover, in periods when information on lemmings was available from potential breeding areas to the north, mass movements of owls coincided with crashes in lemming numbers (Shelford 1945, Chitty 1950). In western North America, irruptions were not well synchronised with those in the east, presumably reflecting asynchrony in lemming cycles between breeding regions (but this has not been proved). The irruptions were also less regular and less pronounced in the west than in the east, with some birds appearing on the northern prairies every winter, and some of the same marked individuals appearing on the same territories in different (not necessarily consecutive) winters (Kerlinger et al. 1985).

In eastern North America, two other vole-feeders, the Rough-legged Buzzard Buteo lagopus and Northern Shrike Lanius excubitor, have irrupted at similar 3-5 year intervals, mostly (but not always) in the same years as Snowy Owls (Davis 1937, 1949, Speirs 1939, Shelford 1945, Lack 1954). Perfect synchrony between the

Table 19.4 Irruptions of Snowy Owls Nyctea scandiaca into eastern United States, 1881-2000

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