An American cliff sparrow in flight (Photo: Don DeBold via Wikimedia Commons)
An American cliff swallow building a mud nest on a wall (Photo: Ingrid Taylar via Wikimedia Commons)
American cliff swallows building nests under a bridge
Cliff swallows feeding their chicks still in the nest (Photo: Marlon Harms via Wikimedia Commons)
Professor Charles R. Brown with some of his collection of stuffed road-killed swallows
The American cliff swallow is best known for its yearly migration between North and South America, traditionally resulting in the annual return of the swallows to Mission San Juan Capistrano in Southern California on March 19. Now it seems they also provide a lesson in the workings of natural selection. A three-decade long study carried out by a husband and wife ornithological team in western Nebraska has, thanks to long years of carefully recording all available data, shown that roadkill has exerted a selective advantage on swallows with shorter wingspans.
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