Massive flocks of birds living in Canada's northern forests sometimes migrate hundreds and even thousands of miles south of their normal winter range, and researchers say they've linked the phenomenon to climate — and seeds.
Birdwatchers have been aware for decades of these "irruptions," as the migrations of immense numbers of birds to areas far outside their usual range are known, but scientists say they've now identified climate patterns as the driving force behind them.
That finding may allow predicting of such events for more than a year ahead, they say.
It's all down to boom-and-bust cycles in the production of the birds' main diet of forest seeds driven by cold winters and variations in rainfall, says study lead author Court Strong of the University of Utah.
"It's a chain reaction from climate to seeds to birds," he says.
Although the researchers focused on irruption migrations of a species known as the pine sisken, a type of finch, they report that many other species also experience them, including cedar waxwings, chickadees, purple finches and red-breasted nuthatches.
It's climate-driven large-scale collapse of tree seed production in northern forests of pine, fir and spruce that drives most of these unusual migrations, they say.
"We've known for a long time that weather was probably important, but prior analyses by ecologists have been unable to identify exactly what role weather was playing in this phenomenon," says study co-author Walt Koenig, an ecologist from the Cornell Lab of Ornithology.
The researchers made use of massive amounts of data recorded by backyard birdwatchers taking part in the citizen science effort Project FeederWatch, supervised by the Cornell lab.
Project volunteers log bird sightings from November through April and have gathered more than 2 million observations of pine siskins dating back to 1989.
"... avid birders across the U.S. and Canada have contributed sustained observations of birds at the same broad geographic scale in which weather and climate have also been observed and understood," says another study author, Julio Betancourt of the U.S. Geological Survey.
Pine siskens, which depend mainly on tree seeds for food, breed in summer in Canadian forests and, if seeds are abundant, will remain there during the winter.
However, in years of poor seed production, they will move south in search of winter habitats with sufficient food, sometimes migrating as far south as the Appalachian mountains, the researchers found.
The study findings may help in predicting such events, they say, as favorable and unfavorable climatic conditions in juxtaposed regions called "climate dipoles" two years prior to an irruption appear to push and pull bird migrations up and down the North American continent.