To get a good look at the effects of climate change, look no further than your own backyard: There might be some unexpected birds among the flock you’re used to seeing hovering around the birdfeeder, lured north by winter’s warmer temperatures. A new study finds that warm-adapted species — birds that prefer the warmer winters typically found in southern states, such as cardinals and Carolina wrens — are now wintering farther north than they did 20 years ago.
One of the best fingerprints of global warming is the poleward movement of the world’s animals. But few studies have looked at more than a single species at a time.
To see what was happening to 38 species of birds across eastern North America, University of Wisconsin-Madison wildlife biologists Benjamin Zuckerberg and Karine Princé analyzed two decades of data from Project FeederWatch. This citizen-science project from the Cornell Lab of Ornithology collects reports of bird sightings at backyard feeders from early November to late April. There are currently more than 10,000 participating sites in the United States and Canada, where volunteers count and record birds at a feeding station over two-day periods throughout the winter.
In the new study, published October 16 in Global Change Biology, the researchers used a subset of FeederWatch data from 1989 to 2011 from sites in eastern North America, concentrating on reports from “core winter” — December 1 to February 8. For each site, they determined the yearly average minimum temperature for that core period. The northern boundary of the range for many North American birds is determined by that minimum winter temperature.
Over the 22 years of the study, the minimum winter temperature at the feeder sites gradually increased. In that time, the birds didn’t just collectively start moving north, but many warm-adapted species that decades ago wintered solely in the south began to spend the colder months of the year farther north, the data shows. “The winter bird communities of eastern North America are increasingly dominated by warm-adapted species,” the researchers note. In addition to Carolina wrens and cardinals, purple finches, eastern bluebirds, red-bellied woodpeckers and other species can also be found wintering farther north than they did in the past.
The birds are getting an assist from the feeders, but the scientists don’t think that supplementing the birds’ diet is driving the change in range because the number of bird feeders in the United States has been relatively unchanged since 1991, hovering around 53 million. Habitat loss could also be driving some changes in bird communities, but these changes tend to be local. The observed alterations were spread across eastern North America, making climate change the most likely suspect behind the observed changes.
Whether the migration of certain species proves to be a positive for those birds or a negative for the communities in which they find themselves isn’t yet clear. But it can’t be assumed that species moving northward will adapt just fine. As this study shows, not all species are moving at the same time, plus there’s things like roads and cities in the way. Migration is just one piece of the very complicated puzzle of climate change survival.