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.*snip*
“This is reproductive isolation, the first step of speciation,” said Martin Schaefer, a University of Freiburg evolutionary biologist.
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.
Schaefer says he doubts that birdfeeders will be around long enough to complete the task, but it's a fascinating prospect nonetheless.