Environment

Birds that haven't adjusted to the realities of a warming world are worse off than their more flexible counterparts, according to a first-of-a-kind study directly linking population declines in birds to climate change.

The findings by Diego Rubolini of the University of Milan, Italy, and colleagues are based on more than four decades of migration observations and population estimates of 98 European migratory bird species.

"Those species that are unable to keep pace with climate change could go extinct within a number of years," says Rubolini.

"It's groundbreaking because it shows that the predictions of the past two to three years - that birds are going to decline because of climate change - are already coming true; it's no longer just a prediction," says Richard Primack of Boston University, who was not part of the study.

The team looked at changes in the average migration time of individual species at different observation points in central Europe and Scandinavia from 1960 to 2006. They then compared these travel times with population trend assessments for each species from 1970 to 1990 and 1990 to 2000.

Birds that started to migrate earlier with increasingly warmer spring temperatures had stable or increasing populations, but those that didn't adjust their travel plans had populations that were in rapid decline.

Missing the feast

The researchers attribute the decline to the birds falling out of sync with peak food supplies at their summer breeding grounds. As global temperatures rise, the greatest warming occurs during the spring - especially in the Arctic, where plants and insects have responded by speeding up their annual life cycles.

Primack predicts similar declines will be found in North American birds and suggests the observed population declines are just the beginning. "As temperatures continue to rise, these changes are going to get a lot more dramatic for birds, mammals, and other groups of organisms," he says.

The study accounted for a number of factors that could also have affected changes in population including; migration distance, location of wintering grounds, and the habitat and latitude of breeding grounds.

Wesley Hochachka, of the Cornell Lab of Ornithology in Ithaca, New York, however, cautioned that population declines may have multiple causes.

Journal reference: Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.0803825105)

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