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Cliff swallows have happily adapted to manmade "cliffs." Photo: < href="">Carol Foil</a>, Creative Commons, some rights reserved
Cliff swallows have happily adapted to manmade "cliffs." Photo: < href="">Carol Foil, Creative Commons, some rights reserved

Natural Selections: Cliff swallow adaptation

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Researchers have found that variations in the wingspan of cliff swallows has a measurable impact on their survival in a human-dominated environment. In this week's Natural Selections, Dr. Curt Stager and Martha Foley discuss how cliff swallows living in a high traffic area have adapted to survive the conditions.

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Summary of this Natural Selections conversation:

Cliff swallows normally make their mud nests on the side of cliffs. Researchers from the University of Tulsa have been studying one group of swallows that makes their nests under manmade structures such as bridges and overpasses. These areas make good shelters that protect mud nests from washing away in the rain. The main problem these swallows face is vehicle traffic in the nesting area, which kills many birds.

The researchers have been at this a long time. Dr. Curt Stager says, “They picked up about 2,000 swallows over almost 30 years.” One thing the scientists noticed was that year after year, they were finding that fewer and fewer swallows had been hit by vehicles.

Their first thought was that the swallow population had decreased. But this was not the case; there was still the same number of nests around the area. Through careful measurements, they found that the birds that had been hit by cars had slightly larger wings than the swallows that had not been hit.

It appears that the cliff swallow population is now adapting to the presence of traffic, though individual birds are not. Birds with slightly longer wings are more apt to be hit by cars, so over the last thirty years more birds with smaller wings have survived long enough to breed. The effect of this selection is that now, the average swallow has smaller wings, making them less likely to be killed by a vehicle.

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