For the first time, researchers have found evidence of a split in the migration pattern of a species of bird, a behavior that some theorize could lead to a new species.

Bands of the European blackcap, which typically breed in Austria and Germany, have begun flying to two separate locations for the winter: one group to Portugal, Spain and North Africa, the other to Britain and Ireland. Scientists studying the two groups found that the birds that wintered together in the north tended to mate with each other once they arrived back in Austria and Germany. These birds also produced more young than those that wintered in the south, which could improve their evolutionary chances of diverging.

"The 'British' birds tend to arrive on the breeding grounds earlier than the southern ones, allowing them to gain access to the best territories--a bit like getting their towels on the best sun-loungers first," said ornithologist Stuart Bearhop of Queen's University Belfast, whose team published its results in the current issue of Science.

The scientists arrived at their conclusions by studying birds from multiple sites over two winters and two summers. In the winters, the team analyzed the ratio of the chemical signatures in claw clippings from the birds, which can be tied to the same signatures in rain from specific regions. They found that the hydrogen signatures measured in those birds from Britain and Ireland were significantly lower than those that resided in Portugal and Spain. Using this knowledge, the scientists spent two summers capturing breeding blackcaps at eight sites in southern Germany and Austria to determine where the pairs had come from.

They discovered that males and females, which had spent their winters together, were 2.5 times more likely to mate than if the birds had mingled randomly--a pairing phenomenon known as assortative mating, which is thought to play a role in the development of new species. Bearhop theorizes that not only are the northern birds prompted by the more dramatic changes in day length to depart for their breeding grounds two weeks earlier than those residing in the south, but they also have a shorter migration route. The early arrivers scoop the prime breeding ground and are potentially more fit to get on with reproduction. What's more, birds that wintered in Britain and Ireland produced one or more eggs than those that had wintered in the south.

Whether the combination of assortative mating and the production of more eggs is causing an evolutionary divergence between the north- and south-wintering birds remains to be seen. But pinpointing the phenomenon is an important step in acknowledging the possibility.