Something as simple as feeding birds can change their biological fate, and even seed the formation of a new species.

Central European blackcap warblers that spend the winter in the birdfeeder-rich United Kingdom are on a different evolutionary trajectory than those that migrate to Spain. The population hasn't yet split into two species, but it's headed in that direction.

"This is reproductive isolation, the first step of speciation," said Martin Schaefer, a University of Freiburg evolutionary biologist.

Blackcap migration routes are genetically determined, and the population studied by Schaefer has historically wintered in Spain. Those that flew north couldn't find food in barren winter landscapes, and perished. But during the last half-century, people in the U.K. put so much food out for birds that north-flying blackcaps could survive.

About 30 percent of blackcaps from southern Germany and Austria now migrate to the United Kingdom, shaving 360 miles from their traditional, 1,000-mile Mediterranean voyage. Because they've less distance to travel, they tend to arrive home first in the summertime and to live in prime forest-edge spots. All this makes the U.K. migrants more likely to mate with each other than with their old-fashioned brethren.

From these groupings, subtle differences are emerging. The U.K. birds tend to have rounded wings, which sacrifice long-distance flying power for increased maneuverability. Now that they don't need wide bills to eat Mediterranean olives in winter, their bills are becoming narrower and better-suited to summer insect diets. They're also slightly darker.

Schaefer thinks it unlikely that humans will keep feeding the blackcaps long enough for them to become truly separate species, but it's possible. He's now studying the fate of hybrid offspring born to British and Spanish migrants, that split the difference between their parents and winter in southwest France. If the hybrids have trouble surviving, the population will likely diverge even further, and now-subtle differences will become pronounced as blackcaps favor their closest kin.

Perhaps the most striking aspect of the findings, published Thursday in Current Biology, is the manner in which the populations diverged, said Schaefer. Reproductive isolation usually begins when a population is separated by a mountain or a sea, as with Darwin's famous Galapagos finches. "Here it's prompted by a very innocent human activity," said Schaefer.

1) A United Kingdom-migrating blackcap at left, and a Mediterranean-migrating blackcap at right./ Martin Schaefer. 2) Beat Walser.

See Also:

*Citation: "Contemporary Evolution of Reproductive Isolation and Phenotypic Divergence in Sympatry along a Migratory Divide." By Gregor Rolshausen, Gernot Segelbacher, Keith Hobson and H. Martin Schaefer. Current Biology, Vol. 19, No 23, Dec. 3, 2009. *

Brandon Keim's Twitter stream and reportorial outtakes; Wired Science on Twitter. Brandon is currently working on a book about ecosystem and planetary tipping points.