When Arctic air and squalling snow send winter temperatures plunging, backyard bird feeders can seem like the trendy bistros of the bird neighborhood—with flocks of sparrows and chickadees cramming in to get their fill. A study published in Journal of Animal Ecology shows that such observations are no coincidence: Some birds flee the countryside and find refuge in urban areas—and the bird feeders there—to survive extreme winter weather.
Ecologists Chris Latimer and Benjamin Zuckerberg used data from over 3,500 Project FeederWatch sites in the eastern U.S. for their research on the connections between winter weather patterns and bird occurrence.
“FeederWatch is great because it provides a huge dataset going back decades, and you get data from the exact same sites across a large geographic area for nearly five straight months. So you can really start to compare when birds are in a certain region and when they’re not,” says Latimer, lead author of the study, who conducted the research as a PhD student at the University of Wisconsin–Madison.
Together, he and his advisor Zuckerberg chose three winters over the past decade for their analysis, including a dreadful winter in 2014–15 that included a polar vortex and the second-coldest February ever recorded by NOAA. Their study included 14 common backyard species, from birds like Northern Cardinal and Tufted Titmouse that have historically lived in temperate areas to cold-weather stalwarts such as Red-breasted Nuthatch and American Tree Sparrow. The researchers also divided the landscape into three categories—forest, agriculture, and urban.
They hypothesized that birds would be more likely to move out of agricultural areas and cities and into forests during harsh winter weather events, because forests can buffer extreme temperature swings. But in the study, species such as Carolina Wren, House Finch, and Northern Cardinal—three of the least cold-tolerant bird species in the study—did the reverse: They moved out of agricultural areas and forested areas and into urban areas during or just after the coldest winter periods.
Downy Woodpeckers, Red-breasted Nuthatches, and Black-capped Chickadees (three species with greater tolerances to cold temperatures) also showed a pattern of moving into urban areas.
“At the end of the day, birds go where the resources are,” says David Bonter, co-director of the Center for Engagement in Science and Nature at the Cornell Lab, who was not involved in the study. He says observations at his own feeders outside of Ithaca, New York, echo some of the research results. For example, in the study American Tree Sparrows flocked to human-populated areas in a big way during cold snaps. And in Bonter’s observations: “No snow, no tree sparrows at my feeders. But the second we see snow in inches, 15 to 20 show up.”
Tree sparrows prefer to eat the seed heads on dead grasses and flowers often found in open areas. But when deep snow covers the ground, the sparrows must look for other sources of food.
Latimer, now with The Nature Conservancy, says bird feeders may be helping birds such as American Tree Sparrows. He says that the urban heat island effect may also be playing a role. Average winter temperatures can be up to 6˚C (10˚F) warmer in urban areas than rural areas. According to Latimer, the combination of plentiful food and more moderate winter temps in cities may help species on the move that historically had more southern distributions.
“As winters become more unpredictable, urban areas may provide a refuge for birds like Carolina Wrens and Northern Cardinals as their ranges are shifting north,” he says.
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