Pine siskins are a North American finch that have long puzzled ornithologists. In astonishing numbers, they will suddenly appear thousands of kilometers away from their normal habitat for a season, before disappearing again for several years. Now, a collaboration between zoologists, climatologists and amateur observers has explained the phenomenon, and possibly given us the tools to predict where they will turn up in the future. The findings could prove important to our understanding of the effects of climate change at high latitudes.
Pine siskins (Carduelis pinus) feed on seeds, buds and some insects. While still common, their numbers are hard to track because of their “irruptive” movements; sometimes they come south in huge numbers into areas where in normal years they are largely absent.
The siskins breed in the pine forests of northern Canada. In years of abundant food, they stay in Canada for the winter, but in bad years they travel as far south as Mexico City to supplement their diet.
The phenomenon, also seen in Bohemian waxwings and hoary redpolls, has been attributed to food supply, but beyond this hypothesis little has been known. Now, a paper in The Proceedings of the National Academy of Sciences reveals that these changes in food supply are driven by seasonal climatic variations, not over a single year but over periods of up to a decade.
Using more than 2 million observations from Project FeederWatch, a multidisciplinary team tracked the year-by-year movements of the birds. These were then compared to years of temperature and precipitation reports. Big migrations, such as those in 1990 and 2009, turned out to be associated with conditions two to three years earlier.
Wet, cold winters are bad for seed production several years later. Across the continent, the team found two dipoles—one running north-south, the other east-west—which meant that when seed was bad in one area it was good at the other end of the dipole. Usually a cold winter in eastern North America was accompanied by a warm one to the west–as seen during the last two years.
"It's a chain reaction from climate to seeds to birds," says lead author Dr. Courtenay Strong of the University of Utah. The authors are worried, however, that global warming may lead to years where conditions are bad everywhere, leaving the birds with nowhere to feed.
The idea that migratory species benefit from adjusting their movements to follow food supplies seems obvious, and is taken for granted in this and much other research. However, the first published evidence of fitness benefits from the approach only appeared this week. In the Journal of Animal Ecology the Australian National University's Dr Dejan Stojanovic showed that Tasmania's swift parrots (Lathamus discolor) follow the food supply, giving them an advantage over sedentary species, a lesson applicable to the siskins, among many others.
Stojanovic previously revealed that the swift parrot is in decline as a result of its eggs being eaten by sugar gliders displaced by logging from their usual home. He's now running a crowd funding campaign to see if glider-proof nesting boxes can rescue the parrot and two other endangered birds experiencing the same threat.
Astonishingly, the citizen science that underpinned this research could run into legal problems in the future. Several states have passed laws to prevent activists from making scientific observations in undeveloped areas in order to prevent the publication of reports indicative of environmental damage. However, according to a story in Slate, Wyoming's statute is so broad it would ban recordings or photographs of bird migrations if they are intended to be shared with the federal or state government and are taken on “open land.” One of the authors of the PNAS paper, Dr. Julio Betancourt, is at the U.S. Geological Survey.
