From various sources, summarised in Newton (2002).

From various sources, summarised in Newton (2002).

three species would perhaps not be expected, because their breeding ranges only partly overlap. The buzzard and the shrike breed mainly in the transition zone between forest and tundra, while the owl breeds on the open tundra, but part of the owl population winters in the transition zone. Nevertheless, with most invasions of each species coinciding with those of the other species, the level of synchrony is striking. In the shrike, the cyclic pattern in invasions was most marked before 1950, after which the fluctuations became more irregular and eventually hardly apparent (Davis & Morrison 1987).

In North America, Northern Hawk Owl Surnia ulula irruptions tend to occur at 3-5 year intervals (modified by snow cover, and in different years in different regions) (Duncan & Duncan 1998). Great Grey Owl Strix nebulosa irruptions are also occasionally recorded south and east of the usual breeding range, with big flights noted in eastern regions in 1978, 1983, 1991, 1995 and 2004 (Nero et al. 1984, Davis & Morrison 1987, Bull & Duncan 1993, Jones 2005), while Saw-whet Owl Aegolius acadicus migrations are also much more marked in some years than in others (National Audubon Field Notes). In Europe, the Great Grey, Tengmalm's Long-eared and Short-eared Owls seem to migrate on regular 3-4 year patterns (Harvey & Riddiford 1996, Schmidt & Vauk 1981, Hilden & Helo 1981, Lohmus 1999), as expected from the 3-4 year crashes in microtone rodent populations. As in seed-eaters, movement often begins earlier than usual in irruption years, and mortality among the participants is often heavy, as illustrated by the 500 Great Grey Owls found dead in Ontario in 2004-2005 (Jones 2005).

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