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Male Barn Swallows Become Sexier Through Notable Evolutionary Shift

By Carl Zimmer

The New York Times -- Swallows are getting sexier.

Male barn swallows that have long tail feathers attract females, and European researchers have observed that over the last 20 years those feathers have become much longer.

“We’ve demonstrated quite a dramatic change in a short period of time,” said Dr. Anders Pape Moller, an evolutionary biologist at Pierre and Marie Curie University in Paris, who conducted the research with Dr. Tibor Szep of the College of Nyiregyhaza in Hungary. The findings are to be published in The Journal of Evolutionary Biology.

Experiments suggest that the males’ tails act as advertising for good genes because males must be in good health to spend the energy growing them. The females are particularly attracted by the tail’s two outer feathers, the researchers say.

Moller, who has been documenting this preference for more than two decades, found that the outer feathers had lengthened by almost half an inch (11.4 millimeters), an increase of 10 percent, one of the biggest evolutionary shifts ever documented in a living population of wild animals. By contrast, the central tail feathers, which do not produce a reaction in females, have not changed.

Over the last 30 years, researchers have made more than 1,500 measurements of natural selection in wild populations of animals and plants. Droughts on the Galapagos Islands, for example, have favored larger beaks in finches because the surviving seeds are harder to crack. But the shift measured by Moller and Szep is greater.

Moller and Szep suspect that the agent for this is the long-term spread of the Sahara. The barn swallows they are examining migrate from Denmark to South Africa for the winter, and then return by way of Algeria in the spring. The reduction of vegetation around the desert may mean fewer insects for the hungry swallows.

“If they get across the Sahara and there’s nothing to eat, it’s tough,” Moller said. Weaker male swallows starve; the stronger ones reach Denmark. There they pass on their genes to the next generation, including genes for longer tails.

But that explanation has prompted some skepticism.

“I am prepared to believe that the birds are probably evolving, but not that climate change is the cause, simply because there can be so much else going on,” said Dr. David Reznick, an evolutionary biologist at the University of California at Riverside.