Irruptive migration of common redpolls

We do not know whether the movements of irruptive migrants are fundamentall y different from more conventional migration of birds to their wintering ar eas because irruptive migration has never been thoroughly described for any species on a continent-wide scale. We use data from a citizen-based monito ring 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 n ew areas during this irruptive migration, the normal wintering range was no t completely abandoned. In fact, redpolls were more prevalent in the southe rn part of their normal wintering range in an irruption year than was typic al in non-irruption years. There also was no indication that the majority o f the North American population of redpolls moved continuously through the winter: although redpolls peaked in prevalence at different times across th e continent, their spring departure was relatively synchronous across the e ntire continent. Group sizes were typically smaller the later redpolls arri ved in a region, also suggesting that the entire redpoll population did not move continuously. in contrast to a non-irruptive migrant, the American Tr ee Sparrow (Spizella arborea), redpolls arrived at feeders in a given regio n later but reached peak densities more quickly. Our results suggest that t he irruptive migration of redpolls is more allied to conventional winter mi gration than to nomadism.