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Environment & Science

Study: Swallows evolved smaller wings to avoid threat of cars

Cliff Swallow ( petrochelidon pyrrhonota) with the early makings of a nest.
Cliff Swallow ( petrochelidon pyrrhonota) with the early makings of a nest.
Photo by Ingrid Taylar via Flickr Creative Commons

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The bells will be ringing today at the Mission San Juan Capistrano to celebrate the return of a bird so legendary it has its own song: 

The St. Joseph's Day Festival to commemorate the swallows of Capistrano has been a tradition at the mission for over a century. The swallows migrate to San Juan Capistrano every year around March from their winter home in Goya, Argentina. In recent years, however, their numbers have been declining, likely due to changes in the landscape from urbanization.

A new study out of the University of Tulsa in Oklahoma seems to suggest that the cliff swallows there are adapting to their modified environments in an interesting way. Researchers found their wings may be getting shorter to help them take get out of the way of traffic.

"We found that by picking up dead birds killed by vehicles over the last 30 years, the wing lengths on these birds has changed, and we're finding many fewer dead birds now than we were finding 30 years ago," said Professor Charles Brown of University of Tulsa in Oklahoma. "So it looks like something is going on in the population enabling these birds to better avoid being hit by cars." 

According to data collected in the study, the swallows' wing span decreased about 10 percent. Though that is not a large number, Brown says it's significant because it's happening in birds.

"It's actually quite a bit as bird morphology goes," said Brown. "Bird morphology is fairly static, it doesn't really change much in response to selection, and this is actually a very dramatic change in a morphological trait."

Though scientific evidence points to how natural selection allowing swallows to avoid being killed by vehicles, there are many other environmental factors that can contribute to such a evolution. For instance, Brown says that a change in the availability of insects, the bird's natural prey, and changes to their habitat are factors to consider. Also, natural selection fluctuates over time, so adaptations we're seeing now could cease to exist in the long term.

"Natural selection is a fluctuating process, and sometimes it may go one way and then go back," said Brown. "Over the long term you don't see a lot of change, but in the meantime you can see how populations are adapting to environmental variability."