Chapter 19 - Irruptive migrations: Owls, raptors and waterfowl

Abstract

This chapter is concerned with two other groups of irruptive birds, namely, those owls and other predators that specialize on voles and other cyclically fluctuating prey species, and waterfowl that depend on ephemeral wetlands. Again, the large-scale movements of these species can best be understood in light of their underlying ecology, which I therefore describe in some detail. 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 is plentiful at the time. Some owl species that exploit sporadic rodent supplies move around mainly within the breeding range (for example, 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 in 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 (for example, short-eared Owl Asio flammeus, long-eared Owl A. otus, Common Kestrel Falco tinnunculus, Northern Harrier Circus cyaneus). The movements of waterfowl are influenced by the fluctuating availability of suitable wetland and associated food supplies. Many individuals breed in 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.