Southward autumn invasions (irruptions) by normally northern seed-eating birds are dramatic but apparently irregular events. Irruptive North American species include Bohemian and Cedar Waxwings, Pine and Evening Grosbeaks, Black-capped and Boreal Chickadees, Red-breasted Nuthatch, Pine Siskin, Common and Hoary Redpolls, Purple Finch, and Clark's Nutcracker. The species perhaps best associated with these occurrences, however, are the Red and White-winged Crossbills. Three major questions are raised by these irruptive migrations: What causes them? Are they really irregular events? Are they synchronized among populations within a species and between species?

Ornithologists generally concur that irruptions are triggered by food shortages, such as failure of the coniferous cone crops over a large geographic area. Analysis by ornithologists Carl Bock and Larry Lepthien of many years of Audubon Christmas Counts indicate that a synchronization of seed crop failures in some high-latitude tree species leads to southward irruptions of species normally dependent on those seeds.

Years of good crops, which presumably result in higher population densities of seed-eating birds, are often followed by years with poor crops. Thus, in a year of crop failure that followed one of abundant seeds, bird populations may be larger than normal. This adds to pressure on scarce food resources and serves as additional impetus to migrate. It appears, then, that seed crop size is the primary cause of irruptions and that large population sizes may sometimes be a contributing factor. However, because many other factors (such as insect abundance during the breeding season) can affect population density in any given year, not all species will be affected synchronously by a seed crop failure that leads to irruptions of some species.

Diurnal and nocturnal raptors that feed on small mammals with cyclic population fluctuations constitute another group of irruptive species which also eat foods that fluctuate from year to year in boreal regions. Among North American species, Rough-legged Hawk, Northern Goshawk, Snowy, Great Horned, and Short-eared Owls are known to irrupt periodically. Two main cycles are recognized in boreal small mammals: a four-year cycle among tundra and grassland rodents, and a ten-year cycle that characterizes snowshoe hares. Why populations of these species explode and crash with these approximate periodicities is not clear, but when they crash the predictable result is a southward irruption of many of their avian predators. As in northern seed-eating birds, problems of food scarcity caused by the crash are often exacerbated by dense raptor populations that resulted from preceding years of relatively high prey abundance. Invasions by Rough-legged Hawks and Snowy Owls often occur in the same year, with about a four year periodicity, because both of these species feed largely on rodents. In contrast, invasions by Northern Goshawks, which feed to a great extent on hares and rabbits, occur roughly in ten-year cycles.

SEE: Population Dynamics; Range Expansion; Bird Guilds; Raptor Hunting; How Owls Hunt in the Dark.

Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.