Species fact sheet by Global Register of Migratory Species - www.groms.de

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Mostly migratory and nomadic; some remain in breeding area all year if conditions allow. Movements not predictable, related in ways not fully understood to abundance of prey species; thought to vary considerably from region to region across polar tundra, and intensity of movements fluctuates annually; Nearctic populations periodically irruptive, many or most leaving breeding areas when lemming numbers crash. Winters irregularly S to C & SC USA, N & C Europe, NC Russia, NW & NE China and Japan, occasionally farther S; on average, immature males winter farthest S, adult females farthest N, with adult males and immature females between. Vagrants recorded as far as Bermuda and Azores.   (del Hoyo J, Elliott A, Sargatal J (eds), 1999)

• digitised by GROMS, after del Hoyo J A Elliott; J Sargatal (eds) (1999) - www.hbw.com,
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