Prevailing wisdom has it that the irregular irruptions of winter finches into southern Canada and the lower 48 states are triggered by food shortages caused by the large-scale collapse of seed production in northern pine, spruce, and fir forests. But it turns out that is only part of the story.
Scientists have pinpointed a climate pattern that likely sets the stage for irruptions — a discovery that could make it possible to predict the movements more than a year in advance.
Many seed-eating boreal species are prone to wander, including Bohemian Waxwing, Boreal Chickadee, Red-breasted Nuthatch, crossbills, grosbeaks, and redpolls. According to a study published in May in the online Proceedings of the National Academy of Sciences, persistent shifts in rainfall and temperature drive boom-and-bust cycles in seed production, which in turn drive the mass migrations of Pine Siskin, the most widespread of the irruptive migrants.
Atmospheric scientist Courtenay Strong of the University of Utah and his colleagues used two million observations of siskins submitted since 1989 by backyard birdwatchers to Project FeederWatch, the popular citizen-science initiative run by the Cornell Lab of Ornithology.
The researchers paired the information with 27 years’ worth of climate data and found that weather patterns in one year could predict the birds’ movements for as much as the next two years.
Moreover, the authors identified two seesaw patterns of climate-induced seed production: from north to south and west to east. When wet and cold conditions in one region produced few seeds, the weather tended to be warmer and drier — and favorable to seed production — in another region. The birds likely followed social cues to areas with plenty of seeds.
“It’s a chain reaction from climate to seeds to birds,” says Strong.
Read the study
Courtenay Strong, Benjamin Zuckerberg, Julio L. Betancourt, and Walter D. Koenig. Climatic dipoles drive two principal modes of North American boreal bird irruption. PNAS 2015 112 (21) E2795-E2802; published ahead of print May 11, 2015, doi:10.1073/pnas.1418414112. Full text.
A version of this article was published in the August 2015 issue of BirdWatching. Subscribe.