Irruptive Migration of Common Redpolls

We do not know whether the movements of irruptive migrants are fundamentally different from more conventional migration of birds to their wintering areas because irruptive migration has never been thoroughly described for any species on a continent-wide scale. We use data from a citizen-based monitoring program, Project FeederWatch, to describe the patterns of movements of redpolls (principally Carduelis flammea) across North America in 1993-1994, the winter of a major irruptive migration. Although redpolls moved into new areas during this irruptive migration, the normal wintering range was not completely abandoned. In fact, redpolls were more prevalent in the southern part of their normal wintering range in an irruption year than was typical in non-irruption years. There also was no indication that the majority of the North American population of redpolls moved continuously through the winter: although redpolls peaked in prevalence at different times across the continent, their spring departure was relatively synchronous across the entire continent. Group sizes were typically smaller the later redpolls arrived in a region, also suggesting that the entire redpoll population did not move continuously. In contrast to a non-irruptive migrant, the American Tree Sparrow (Spizella arborea), redpolls arrived at feeders in a given region later but reached peak densities more quickly. Our results suggest that the irruptive migration of redpolls is more allied to conventional winter migration than to nomadism.