In northern regions, microtine rodent densities normally fluctuate on an approximate 3-5 year cycle, but the peaks occur in different years in different regions. The raptors that depend on cyclic rodents may breed or winter in widely separated areas in different years, wherever prey are plentiful at the time. Some owl species that exploit sporadic rodent supplies move around mainly within the breeding range (e.g. Tengmalm's Owl Aegolius funereus, Northern Hawk Owl Surnia ulula), but in other owl and raptor species, part of the population migrates to lower latitudes for the winter, thereby avoiding the worst effects of snow cover. These birds return to the breeding range each spring, settling wherever voles are plentiful at the time (e.g. Short-eared Owl Asio flammeus, Long-eared Owl A. otus, Common Kestrel Falco tinnunculus, Northern Harrier Circus cyaneus). In years of widespread food shortage, some rodent predators leave their breeding range in large numbers, appearing in more southern areas as irruptions. In eastern North America, irruptions of Snowy Owls Nyctea scandiaca, which have been well documented since 1880, have occurred every 3-5 years, at a mean interval of 3.9 years. Similar 3- to 5-year periodicity has been noted in the movements of some other rodent-eating owl and raptor species in parts of North America and Europe.

In North America, Snowshoe Hares Lepus americanus fluctuate greatly in numbers, with roughly 10-year periodicity, but the cycle is more or less synchronised over the whole boreal region. Irruptions of Northern Goshawks Accipiter gentilis and Great Horned Owls Bubo virginianus, which prey upon Snowshoe Hares, have occurred for 1-3 years at a time, coinciding with known lows in the hare cycle.

The movements of waterfowl are influenced by the fluctuating availability of suitable wetland and associated food supplies. According to the types of habitat they occupy, species show varying degrees of site-fidelity and, in some species, distribution patterns change markedly from year to year. This is especially true of species that occupy shallow, ephemeral waters, which disappear altogether in dry years. Many individuals breed or winter in widely separated areas in different years, and migrate further in some years than in others. These various species provide further striking evidence for the importance of prevailing conditions in influencing movement patterns.

Part Four

Evolution of Movement Patterns

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Blackcap Sylvia atricapilla, used in experiments on the heritability of migration behaviour
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