Various species of owls have also been recorded nesting well outside their regular range in years of abundant food, often following invasions (Duncan & Duncan 1998). For example, several hundred pairs of Snowy Owls Nyctea scandiaca bred on the tundra of Swedish Lapland in 1978, where they had been rare to nonexistent in many previous years (Andersson 1980). Snowy Owls bred in Finnish Lapland in 1974, 1987 and 1988, but before these dates none had been seen for several decades (Saurola 1997). Similarly, Northern Hawk Owls Surnia ulula bred in an area in Norway in the peak years of only four out of seven observed vole cycles (Sonerud 1997). This lack of response may arise because in many years the entire owl population can be accommodated in certain parts of the range with abundant prey, without needing to search out other parts. In Fennoscandia, the numbers of Snowy and Hawk Owls at any time seems to be influenced not only by the occurrence of a rodent peak, but by the arrival of large numbers of immigrants from further east (Sonerud 1997). In more central parts of the range, the owls may exploit a much greater proportion of the rodent peaks. When voles were plentiful, Hawk Owls have also bred in invasion areas well south of their usual breeding range in North America (Duncan & Duncan 1998). Among diurnal raptors, numbers of Rough-legged Buzzards Buteo lagopus, Common Kestrels Falco tinnunculus and Pallid Harriers Circus macrourus were also found breeding outside the regular range in localities where voles were plentiful (Galushin 1974).
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