by Mark Derewicz
Every year red-eyed vireos feel the cold pinch of autumn, notice the summer supply of bugs dwindld Brazil. Come springing, and head south for the winter. Most wind up in Ecuador, Peru, Colombia, antime, they return to the United States and Canada. Sounds like a fun life, but timing their migration is serious business.
“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.
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.
Read the paper in PLoS One here and the full article about Allen’s work here at Endeavors