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Small birds that once fled the snow and cold of winter are more likely to stick around thanks to warmer temperatures in northern climates, two UW-Madison researchers have found.

Cardinals, chipping sparrows, Carolina wrens — they’re some of the birds commonly seen in backyards throughout winter that used to skip town.

“These species are moving northward as winters become less severe,” said Benjamin Zuckerberg in an interview Wednesday.

He and co-author Karine Prince, also a UW-Madison wildlife biologist, looked at observations collected from 1990 to 2012 by citizen scientists through the FeederWatch project at Cornell University.

Project volunteers periodically count each bird species they see at backyard feeders between November and early April and report the counts to project organizers. Zuckerberg was a research associate with the project from 2008 to 2011 before joining the faculty at UW-Madison.

The data Zuckerberg and Prince used came from backyard observations in the eastern half of the U.S., from Texas to the Atlantic Ocean, including Wisconsin and other Midwestern states. It stretches from the southern tip of Florida into lower Canada.

The era saw climate changes including warmer average temperatures and less snow cover, particularly during early spring. Zuckerberg and Prince noted subtle changes related to the climate in some of the 38 species of birds included in the study.

“We conclude that a shifting winter climate has provided an opportunity for smaller, southerly distributed species to colonize new regions,” the authors wrote in a paper to be published in the journal “Global Change Biology” out this week.

Yet to be determined is how the newly brave birds fare when winters such as last year’s hit, with prolonged stretches of subzero temperatures. Zuckerberg said some data and studies already suggest the newcomers die off at unusually high rates during extreme cold snaps.

Prince said the shift northward has not been seen at a similar rate for birds that have always wintered in the north as those that are relatively new to the practice.

In the report, Zuckerberg and Prince acknowledged that climate change “should not be viewed as the sole driver” of changes in winter bird populations.

“It would be useful to explore other environmental changes such as shifting land-use practices,” they wrote.

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