By Lauren Morello
The American Redstart has an enviable calendar. The tiny songbird spends its winters in Central and South America, then beats the sweltering heat of tropical summers by flying north to breed.
For scientists, the globe-trotting habits of migratory species like the Redstart pose an immense challenge: As the climate warms, how do you protect a species with no fixed address?
“The difficulty with migrating species is that instead of having to worry about one piece of habitat, you typically have to worry about three,” said Stuart Pimm, a conservation biologist at Duke University. “Where they are in the summer, where they are in the winter, and the places they stop off along the way. And all of those places may be changing.”
A male American redstart.
Credit: Dan Pancamo/Smithsonian.
Rising temperatures and declining rainfall in North Africa have reduced the grazing grounds of the addax, a nomadic antelope. In the Arctic, the sea ice that narwhals and bowhead whales rely on for breeding and protection is receding. And in Eurasia, sea level rise is encroaching on the mudflats and coastal estuaries where the swan goose spends its winters, the Zoological Society of London found.
Closer to home, recent droughts and deforestation have reduced the number of Monarch butterflies spending their winters in Mexico.
In the American Redstart's case, there's evidence that drier winters in the warbler's tropical wintering grounds produce fewer insects for the birds to eat. Fewer birds survive the trek to the Redstart's northern breeding areas. Those that do make it are smaller and weaker.
Pete Marra, a research scientist at the Smithsonian Conservation Biology Institute, said some recent studies suggest that climate change will make those dry winters more frequent, a development that could be bad news for the American Redstart.
But a larger concern, he said, is that scientists don't have much information on the traveling habits of many migratory species, a category that includes everything from whales to butterflies.
The American Redstart, a migratory songbird, travels widely during its annual migration.
Scientists worry that climate change will reduce the birds' winter food sources,
creating new conservation challenges.
Click image to enlarge. Credit: Megan Gnekow © 2010 Cornell Laboratory of Ornithology.
In general, governments have ignored everything but the condition of breeding grounds when trying to determine what threats a migrating species faces.
“For many animals, that means you're missing nine months of their year,” said Marra, who discusses those concerns in a commentary published Sunday in Nature Climate Change. “You can't just look at breeding grounds to understand everything that's happening.”
And sites that are key stopping points for migrating animals may not be useful the next day, points out David Wilcove, a conservation scientist at Princeton University.
For birds, bats and other high-flyers, “what works today, during a period of calm nights, may not work tomorrow when there are storms coming through,” he said. “And what works under today's climate may not work very well in the climate of 20 years from now.”
But there is hope, in the form of new technology, for scientists seeking to shrink gaps in their understanding of species movements.
Researchers are using tiny sensors, the smallest weighing half as much as a dime, to track birds' movements, said Tom Will, a biologist with the U.S. Fish and Wildlife Service.“We're getting new information really quickly now, because of the development of smaller and smaller sensors,” he said.
The devices, which can be strapped to a bird's leg or positioned on its back using tiny harnesses, track changing light levels. Each sensor can record the timing of a year's worth of sunrises and sunsets, data that scientists can use to reconstruct a bird's journey—if they can trap the animal again to remove the device.
They're still not small enough for the smallest birds, like the Redstart, which weighs about a third of an ounce. But as technological know-how increases, the sensors keep shrinking, expanding the range of animals scientists can track.
The little birds are doing well, for now, Marra said. Rainfall in their tropical winter home varies from year to year, so down-and-out dry years are balanced by wet years when insects are abundant.
But scientists remain concerned for their future well-being, and the fortunes of other species navigating long journeys in a changing climate.
“If we want to understand how vulnerable individual species are, we need to take a much more holistic approach in our thinking about how they're going to be exposed to climate,” Marra said. “You can't just look at conditions in the breeding grounds to understand everything that's happening.”