Photos and composite image of birds visiting a feeder in Trenton Falls, New York by Pamela Karaz.
From the winter 2021 issue of Living Bird magazine. Subscribe now.
When the arctic air and stormy snow plummet winter temperatures, backyard birdhouses can feel like the trendy avian bistros – with flocks of sparrows and chickadees crowding to fill up. A study published in the Journal of Animal Ecology shows that such observations are no accident: some birds flee the country and find refuge in urban areas – and the birdhouses there – to survive extreme winter weather.
Ecologists Chris Latimer and Benjamin Zuckerberg used data from over 3,500 Project FeederWatch locations in the eastern United States to study the links between winter weather patterns and bird populations.
“FeederWatch is great because it has a huge data set that goes back decades and gives you nearly five months of data from the exact same locations in a large geographic area. So you can really start comparing when birds are in a certain region and when they are 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 selected three winters for their analysis over the past decade, including a terrible 2014/15 winter with a polar vortex and the second coldest February ever recorded by NOAA. Their study included 14 common backyard species, from birds like the Northern Cardinal and Tufted Titmouse, which historically lived in temperate areas, to cold-weather stalwarts like the Red-breasted Nuthatch and American Tree Sparrow. The researchers also divided the landscape into three categories: forest, agriculture, and urban.
Black-capped Tit by Kittie Wilson. American Goldfinch by Mark Archibald.
They hypothesized that birds would be more likely to move from agricultural areas and cities to forests during harsh winter weather events, as forests can cushion extreme temperature fluctuations. In the study, species like Carolina Wren, House Finch, and Northern Cardinal – three of the least cold-tolerant bird species in the study – did the opposite: they moved during or out of agricultural and wooded areas to urban areas shortly after the coldest winter periods.
False woodpeckers, red-breasted nuthatches, and black-capped chicks (three species more tolerant of cold temperatures) also showed a pattern of movement into urban areas.
“Ultimately, birds go where the resources are,” says David Bonter, co-director of the Center for Engagement in Science and Nature at Cornell Lab, who was not involved in the study. He says observations on his own feeders outside of Ithaca, New York reflect some of the research. For example, in the study, American tree sparrows streamed into human-populated areas on a large scale during cold spells. And in Bonter’s observations: “No snow, no tree sparrows on my bird feeders. But the second we see snow in inches, 15-20 show up. “
Tree sparrows prefer to eat the seed heads on dead grass and flowers, which are often found in open areas. But when deep snow covers the ground, the sparrows have to look for other sources of food.
Latimer, now at The Nature Conservancy, says birdhouses could help birds like American Tree Sparrows. He says the urban heat island effect may also play a role. Average winter temperatures in urban areas can be up to 6 ° C warmer than in rural areas. According to Latimer, the combination of abundant food and moderate winter temperatures in cities can help species move that historically had more southern distribution.
“When winters become more unpredictable, urban areas can provide refuge for birds like Carolina Wrens and Northern Cardinals as their range shifts north,” he says.