Owls and raptors that depend on cyclically fluctuating prey species often suffer food shortages in years when their prey numbers crash. In some northern species this leads to massive southward emigration. Two main prey systems underlie their behaviour: (1) an approximately 3-5 year cycle of small (microtine) rodents in the northern tundras, open parts of boreal forests and temperate grasslands;1 and (2) an approximately 10-year cycle of Snowshoe Hares Lepus americanus in the boreal forests of North America (Elton 1942, Lack 1954, Keith 1963, Stenseth 1999). The numbers of certain grouse species also fluctuate cyclically, in some regions in parallel with the rodent cycle and in others with the longer hare cycle (Hornfeldt 1978, Keith & Rusch 1988, Newton 1998b). Any predators that specialise on such prey species, as well as more generalist predators that also eat other things, are affected by the fluctuations in these prey.
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