Early spring.
Barn swallows and other long-distance migrant birds are returning earlier to their Northern European breeding grounds as a result of climate change.

Earlier Return for Long-Haul Migrants

Climate change has led several migrating bird species to re-set their travel clocks. In a new study, researchers claim that the earlier onset of spring in northern Europe seems to affect long-distance bird migrants, most of which winter in Africa, more than it does Europe's short-distance avian vacationers. Researchers say the findings suggest that these birds' earlier migration may result from evolutionary changes driven by the warming climate.

Previous studies have confirmed that birds have been arriving in Europe earlier in the spring than they once did. (ScienceNOW, 5 May and 3 October 2003). Most of the research, however, suggested that bird species that migrate short distances would be most likely to adjust their flight schedules, because these birds can keep tabs on climate changes in their summer grounds from their nearby winter habitats. In contrast, most researchers doubted that birds wintering as far away as Africa would be similarly driven to an early return.

Now, a team of scientists has shown that they do. After analyzing 24 years of data for more than 30 bird species, the team found that some tropical travelers are arriving several days earlier than they did in the 1980s--a change that is greater than that exhibited by the short-distance migrants, the researchers report 30 June in Science. While researchers cannot rule out the possibility that the earlier migration relates to foraging conditions where they winter in Africa, ecologist and lead author Niclas Jonzén of the University of Lund in Sweden says the better explanation is that birds migrating from Africa have experienced evolutionary changes "based on selection pressure for early breeding in Europe." As spring arrives more quickly in northern Europe, food sources also emerge earlier. Thus, birds that can migrate and breed earlier in the spring may have an advantage over birds that do not adapt as the climate changes.

Environmental scientist Tim Sparks of the Centre for Ecology and Hydrology in Huntingdon, United Kingdom, has misgivings about attributing the earlier migration to evolutionary changes and believes more data are needed from the southern hemisphere. Still, he says the study is a step in the right direction. "Many bird species are undergoing big declines," says Sparks. "Undoubtedly better information on the effects of climate on bird migration is needed to predict future changes in biodiversity."

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