Every year red-eyed vireos feel the cold pinch of autumn, notice the summer supply of bugs dwindling, and head south for the winter. Most wind up in Ecuador, Peru, Colombia, and Brazil. Come springtime, they return to the United States and Canada. Sounds like a fun life, but timing their migration is serious business.

Click to read photo caption. © Bill Hubick

“They have to time their arrival at breeding grounds just right,” says biologist Allen Hurlbert. “If they come too early, they might face bad weather conditions and limited food resources. If they come too late, they might struggle to establish breeding territories. They have to arrive at the peak time when trees and plants are leafing out and bugs are plentiful.”

So what happens when spring starts coming earlier? Can the vireo adjust? Can other birds? It depends on the species, Hurlbert says. Some are responding better than others. But even the ones adapting well right now could have trouble thriving if temperatures continue to rise. If birds can’t adapt to climate change, then there will be consequences for humans.

Birding is a popular pastime in the United States. Since 2002 more than 35,000 people equipped only with a pair of binoculars and a reference book have posted over 48 million observations to a database called eBird. Anyone can download those observations, and over the past few years scientists have started studying the data. Hurlbert, a longtime birdwatcher, downloaded information for the 18 most common migratory bird species in the eastern United States to study when certain species make their first appearances at their breeding grounds. Then he plotted those findings against temperature records from 2002 to 2010. He found that birds reached their breeding grounds earlier the warmer the weather. But some species, Hurlbert says, were able to adapt to annual shifts in temperature better than others. And that means they arrived at breeding grounds at more optimal times than did the species that couldn’t adapt their migration schedules as well.

Click to read photo caption. © Martin Mecnarowski

Birds have several traits that determine how well they can respond to temperature changes. Hurlbert found that the most important one is the speed at which birds migrate. Typically, the slower a species migrates the better it can adapt to temperature changes. Some birds, such as the vireo, can adjust on the fly. If the weather is cooler than normal or if food is plentiful along their migratory paths, then they may stay farther south before making their way to their breeding grounds. Others, like some barn swallows, leave South America and fly as quickly as they can to their springtime breeding grounds.

Why some birds do this better than others is a bit of a mystery, Hurlbert says. “You can put some bird species in a room with no variation in light or temperature, and they’ll exhibit what we call migratory restlessness: they act like the time has come to migrate even though there aren’t any environmental cues.” They don’t rest at night as much and they eat a lot more than usual to accumulate fat deposits that serve as fuel during long flights. Barn swallows experience this restlessness. They seem to have a strong internal clock, which Hurlbert says may explain why they can’t easily alter their migration schedules. Other species can adapt to their surroundings. For example, red-eyed vireos are able to adjust their migration timing while in transit. This adaptability, Hurlbert says, helps them arrive at breeding grounds when the temperature is ideal and the number of insects is optimal for feeding their young.

Hurlbert thinks this ability to adjust may be the reason the vireo has seen its population increase over the past several decades. The house wren is another bird that can alter its migration and whose population is increasing, he says. Other birds, such as the barn swallow and eastern wood-pewee, show very little ability to shift their migration timing in response to changes in spring temperatures. Both of their populations are declining.

Still, Hurlbert points out that temperatures shift from year to year for reasons that have little to do with climate change. “But if the mean temperature keeps rising and a particular species isn’t responding well, he says, then there will be more years on average when that species is arriving at breeding grounds at inopportune times. And if that happens more and more frequently, then that will lead to population decline.” The ultimate fear is that some species could face extinction over the next fifty to a hundred years, Hurlbert says. That’s why it’s important to assess their adaptability now, even for the most common species of birds and other animals, such as the insects birds eat.

What bird extinctions mean for humans or the ecosystem at large is hard to foretell. Losing one animal species might not cause a major change or a ripple effect that hurts humanity. But Hurlbert says that the more species that go extinct, the more likely it is that our ecosystems will begin to fail. “Barn swallows and purple martins—those birds eat gazillions of flying insects every year,” Hurlbert says.

Click to read photo caption. Dori

A lot of those bugs are mosquitoes and crop-killing pests. Would other birds or animals pick up the slack? What kinds of birds? What traits would they possess that could affect their ecosystems? Would insects eat more leaves, kill more trees, ruin the wildflowers that bees depend on? It’s hard to know, Hurlbert says. There could be other cascading effects that are difficult to fathom, let alone predict.

He’s also quick to point out that birds that migrate slowly might not be off the hook.  Some species, like the great crested flycatcher, are shifting their arrival times earlier in the southern United States than in the North. Hurlbert’s projections show that later this century these southern-breeding birds will arrive in the South a week earlier than they are now, on average. But northern-breeding birds are projected to arrive at their breeding grounds only two or three days earlier than they do now. That means that northern-breeding birds will be spending more time in transit, putting more strain on resources along migratory routes in the South.

No one knows for sure how all this will play out, though Hurlbert’s work sheds light on how birds are reacting now and what they might face in a warmer world. None of the 18 species he studied are in any danger right now, but his work has given conservationists another tool for assessing the threat of climate change. Conservationists want to raise awareness before a species gets close to the endangered list.

“I hope this research points the way to a better understanding of biological processes,” he says. “This study linked arrival dates to temperatures. But it might not be temperatures that birds are cuing in on. It’s certainly not temperature alone.”

Allen Hurlbert is an assistant professor of biology in the College of Arts and Sciences. His paper was coauthored by former UNC undergraduate Zhongfie Liang. Their study was published in PLoS ONE.