A new study based on the National Audubon Society's North American Christmas Bird Count finds birds have taken decades to adjust their ranges northward in response to warming winters.
Frank La Sorte, a post-doctoral researcher at the Cornell Lab of Ornithology in Ithaca, was lead author of the study published online this month by the Journal of Animal Ecology. He said animals adjust to rising minimum winter temperatures by shifting their ranges northward. Since birds are highly mobile and migrate north and south with the changing seasons, they're better able to shift their ranges than less-mobile, non-migrating species, like amphibians.
But the study of 59 bird species found it's not all that easy or quick. And some birds are better equipped to follow the changing climate than others.
Take black vultures. While the minimum winter temperature increased from 1975 to 2009, it took black vultures 35 years to catch up with the trend. Over that time, they have spread northward as far as Massachusetts, where winters now are similar to Baltimore's in 1975.
On the other hand, the endangered red-cockaded woodpecker hasn't moved at all. La Sorte said that could be because they have such specialized habitat needs, found only in the sandy longleaf pine forests of 11 southern states. "It might be good to go in and look at how well they're coping" with the rising temperature, La Sorte said Tuesday in an interview. "It depends on their physiological tolerance, and changes in the prey base."
La Sorte said species that don't track climate changes may wind up in habitats that don't suit them well.
"When you think about it, it makes sense that species move slower than the rate at which climate is changing," La Sorte said. "They're not just tracking temperature; many of them need to follow a prey base, a type of vegetation, or they need certain kinds of habitat that will create corridors for movement."
There are also species, such as turkey vultures and ruby-throated hummingbirds, that moved north faster than the temperature warmed.
"The take-home message is that as species are responding, they're doing it under their own time frame," La Sorte said. "Understanding what that time frame might be is challenging. We have to give species the opportunity to respond by providing corridors for movement, and long-term maintenance of those corridors. That would require cooperation across political boundaries."
Such a corridor might be a swath of forest or grassland that extends northward so birds can gradually move north along it, rather than being stopped by a sprawling metropolitan region.
"There's a lot of discussion about it, but it's primarily academic," La Sorte said.
The study, co-authored by Walter Jetz of Yale University, was supported by the National Science Foundation.