Bird species trace their migratory paths based on where they can find the most resources, but changes to the climate might soon make their knowledge outdated.
In a recent study, a team of scientists from the University of Copenhagen tracked the movement of three species — the common cuckoo, the red-backed shrike and the thrush nightingale — to identify what prompted their annual migratory routes.
They found that the birds tend to gravitate to places that had the best supply of food, like insects, at different times of the year. They also keep an eye on local vegetation, moving toward verdant places in what researchers call “green wave surfing.”
Birds could soon have a tougher time doing that. Climate change is expected to change vegetation patterns and food supplies in different parts of the world, and birds might find that they're unable to find the best stopovers along their route.
“If you look at the migration routes and schedules they have now, and then at climate models for the end of the century, the birds aren't faring well,” said Kasper Thorup, with the University of Copenhagen's Center for Macroecology, Evolution and Climate. “They expect areas to be green when they arrive, because for many years, they have been following an innate program. But these areas could be less green in the future.”
The seasonal journeys that birds make can be extremely long and complicated, with some of them crisscrossing continents and traveling thousands of miles to find the right spot to breed.
According to Thorup, certain species make five to seven stopovers along the way. Some last for over a month, so tracking their movements is complicated. The team used data collected from birds fitted with tracking bands to get a better idea of their routes.
An analysis of the data indicates that birds travel long distances based on an innate knowledge of food availability, said Carsten Rahbek, co-author of the study.
“Our results suggest that by the end of this century climate change, and other impacts on the food source, like land use changes, could negatively influence the birds' chances to find sufficient food,” he added in a statement.
According to Allen Hurlbert, associate professor with the University of North Carolina's biology department, there are two ways that changing climate patterns can affect these birds: by directly altering the environment they are used to breeding in and by changing the amount of resources and food available in those places. Scientists are concerned about the overall food availability and its timing.
“If the bird isn't aware of that change in timing and it migrates at its usual time, it may be nesting and trying to raise young at a time when the peak in food availability has passed,” he explained. “That's a challenge and a concern — that birds are mistiming their migration.”
Certain birds tend to be more vulnerable to these changes than others. For instance, birds that typically migrate longer distances are more prone to falling out of key with resource fluctuations, said Frank La Sorte, research associate at the Cornell Lab of Ornithology, because they have a more rigid migration schedule. Short-distance migratory birds, on the other hand, are already in the region they're traveling to and can make calculations based on the conditions they're experiencing.
“Long-distance migrants leave at the same time every year. If there's a dramatic change in the timing of resource pulses in the breeding grounds, it's going to result in population declines,” he said.
Species that have more specialized and rigid diets tend to be at greater risk, too, because there's a higher likelihood they won't be able to find the food they need while breeding.
Thorup views this study as a first step toward understanding more about how birds migrate, which is essential to figuring out conservation strategies. The priority going forward, he said, will be to figure out important spots that the birds depend on during the long journey.
“We need to be on the ground to find out which areas are critical — areas, for example, where the birds are staying around and fattening up before traveling are still unknown. This will give us the knowledge we need to guide what research has to be done,” he said.
Reprinted from ClimateWire with permission from E&E News. E&E provides daily coverage of essential energy and environmental news at www.eenews.net.
