For several years, scientists have suspected that climate change is driving birds to migrate earlier each spring. In fact, we reported just recently that Ruby-throated Hummingbirds are showing up on their breeding grounds 11-15 days earlier than they had a few decades ago.
The same thing is happening with Black-tailed Godwits in Iceland. The first godwits return to the island in mid-April from wintering areas that extend from Ireland to Morocco; in the late 1980s, the birds began to arrive in late April or early May.
A study published today in the British journal Proceedings of the Royal Society B reveals that individual godwits migrate like clockwork — arriving at the same time each year. So how is it possible that the spring arrival date has advanced by two weeks?
It’s the younger birds — those hatched in the last decade or so — that are migrating earlier, the researchers say.
“Because we have been following the same birds for so many years, we know the exact ages of many of them,” says lead author Jenny Gill, an ecologist at the University of East Anglia and the president of the British Ornithologists’ Union.
“We found that birds hatched in the late 1990s arrived in May, but those hatched in more recent years are tending to arrive in April,” she says. “So the arrival dates are advancing because the new youngsters are migrating earlier. Climate change is likely to be driving this change because godwits nest earlier in warmer years, and birds that hatch earlier will have more time to gain the body condition needed for migration and to find good places to spend the winter, which can help them to return early to Iceland when they come back to breed.”
The finding may also explain why advances in migration timing are not common among species that migrate longer distances. “Many long-distance migrants arrive so late on the breeding grounds that they have little opportunity to respond to warming conditions by nesting earlier,” Gill adds. “This research is very important because many long-distance migrant bird populations are currently declining very rapidly, and identifying how climate change is affecting these populations is a key part of understanding the causes of these declines.” — Matt Mendenhall, Managing Editor
Read the paper
Jennifer A. Gill, José A. Alves, William J. Sutherland, Graham F. Appleton, Peter M. Potts, and Tómas G. Gunnarsson, Why is timing of bird migration advancing when individuals are not?, Proceedings of the Royal Society B, November 13, 2013.