The study models a range of scenarios with a range of greenhouse gas emissions and predicts how the distribution of nearly 600 North American bird species will change because of rising temperatures. Even under the most hopeful scenario, the study found that 126 species will lose more than 50 percent of their current range––with no opportunity to expand to more hospitable locations. In some cases, species will lose almost 100 percent of the area suitable for their survival.
The results were sobering and more dire than expected, said lead author Gary Langham, chief scientist of the National Audubon Society.
"It wasn't as if the ranges were just shifting but rather they were just disappearing, almost the way you think about species on a mountaintop being pushed further and further up the top of a mountain," Langham said.
The study found 188 species that will lose more than half of their range, but could potentially inhabit new areas.
"The Baltimore Oriole, state bird of Maryland and mascot for Baltimore's baseball team, may no longer nest in the Mid-Atlantic, shifting north instead to follow the climatic conditions it requires," Langham said in an email.
The ability of some birds to colonize new areas is encouraging but other factors may make relocation difficult.
"Just because climatic space moves doesn't mean that all the things they need on the ground like habitat and food and all the predator interactions won't necessarily line up in a favorable way for them in the future," Langham said.
The study relied on data from Audubon's annual Christmas Bird Count and the U.S. Geological Survey's Breeding Bird Survey. Combined, those findings provide snapshots of places where hundreds of bird species are found in winter and early summer. Langham and colleagues then combined this information with a number of climate variables, including temperature and precipitation, to build a profile of the range of climatic conditions in which individual species live.
"You can then project the possible shift [in geographic range] onto any future climate that you have," Langham said.
Models based on climate are a start, but the true picture can be much more complicated, said Amanda Rodewald, director of conservation science at Cornell University's Lab of Ornithology.
"We know species are part of a community and they are influenced by other species [like] certain plants that they depend upon," Rodewald said.
Allen Hurlbert, a biologist at the University of North Carolina, agreed, citing birds living in the boreal forests that extend across south and central Canada as an example. If the temperatures rise, the birds might move farther north for a cooler climate––but doing so would push them into a completely different, tundra habitat.
"Are they going to do less well where the climate is right but there are no food resources and no habitat or are they going to do less well where the climate is slightly uncomfortable but there is all the habitat and food they are accustomed to?" Hurlbert asked.
Rodewald added that focusing too much on climate change could come at a cost.
"We don't want to lose sight of the fact that right now today we are still living in a world where habitat loss and degradation are the most serious threats imperiling species around the globe," she said.
Rodney Siegel, executive director of the Institute for Bird Populations in California, praised the work as an important first-of-its-kind study.
"This is a starting point," Siegel said. "What you see here is raw material for many, many specific hypothesis about how individual birds in particular places are going to move around on the landscape. What I would like to see happen is a flourishing of follow-up studies."
At the same time, Siegel says, it's time to start acting on what we already know about the effects of a warming planet.
"It's kind of a wake-up call that many of our more common species even could be in jeopardy in the coming decades," Siegel said. "We need to get land management right to do the best conservation that we can in the face of climate change that is largely inevitable."