Strangers in Your Backyard? Thank Climate Change

This winter, your backyard feeder is likely to be populated by a very different array of species than it would have been just 20 years ago. And for that, you can thank global warming. As temperatures warmed throughout eastern North America over the past two decades, many bird species have expanded their winter ranges northward. A recent study published in Global Change Biology shows that these expanded ranges are bringing new combinations of bird species together.

Many individual species have altered their movements in response to climate change, but it’s been unclear whether these isolated shifts are broadly affecting backyard communities. To assess this, Karine Princé and Benjamin Zuckerberg, wildlife biologists with the University of Wisconsin-Madison, used bird counts taken between 1989 and 2011 by Project FeederWatch—an international volunteer program in which citizen scientists count and record the number and species of birds gathered at their backyard feeders—to analyze winter communities across eastern North America. These overwintering bird species, which are adapted for survival at low temperatures, have been long recognized as a sensitive indicator of environmental change, such as climate. 

The volunteer monitoring program, coordinated by the Cornell Lab of Ornithology and Bird Studies Canada, runs from November through April and provided the team with data on the relative abundance of 38 bird species at over 30,000 eastern North American sites, ranging from Newfoundland, Canada south to Florida and west to Texas. In their analysis, Princé and Zuckerberg discovered that the winter bird communities of eastern North America have been slowly changing, with bird species adapted for warm climates, including Carolina Wrens and Chipping Sparrows, becoming increasingly common further north than normal. The changes to bird communities were most pronounced in the lower United States, and generally associated with warming winter temperatures. 

“Climate change is occurring, species are responding to that change, and we are starting to see those changes in our own backyards,” says Zuckerberg. 

The study comes on the heels of Audubon’s Birds and Climate Change Report, which used citizen science data from annual Audubon Christmas Bird Counts and North American Breeding Bird Surveys to predict the future impacts of climate change on North American birds. The report concluded that more than half of North American bird species are in danger of losing 50 percent or more of their current geographic range due to changes in climate over the next century. Princé and Zuckerberg’s research suggests this is already happening, and entire bird communities are changing as a result. 

“The [Audubon] models tend to support our general findings,” Zuckerberg says. “It’s likely that the reshuffling of species will continue, and the future bird communities will look very different than those of today.”

Princé and Zuckerberg caution that climate may not be the sole driver of changes in winter bird assemblages. Other factors, such as habitat loss as a result of urban development, may also contribute. And whatever the cause, the ecological consequences of the bird community’s shift remain unclear. “Many of the changes being observed are unprecedented,” Zuckerberg says. “We know from other studies that when you start forming novel communities, we lose the ability to predict how they are going to respond to environmental change.”