Feeding birds could create 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 some 360 miles from their traditional, 1000 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.