Its distribution is irregular, it being abundant at one season and almost totally unknown the next. (E. W. Nelson 1887, writing of Snowy Owls.)
This chapter is concerned with two other groups of irruptive birds, namely those owls and other predators that specialise 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. Ducks are not usually regarded as irruptive migrants but, particularly in arid regions, they show many of the typical features: a tendency to concentrate temporarily wherever conditions are suitable, and in widely separated areas in different years. In winter in many regions they are also greatly influenced by prevailing temperatures, as individuals move to avoid areas where surface waters are frozen, appearing in numbers in the furthest parts of their wintering ranges only in exceptional years.
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