Winter Neighbors, Summer Lovers

European blackcaps who spend the winter together are more likely to mate together during the summer, a new study finds.

During the summer months, European blackcaps like the one pictured above breed in south central Europe and then migrate to southern Spain and North Africa for the winter. In recent decades, however, some blackcaps have begun to break from tradition. Instead of flying south for the winter, they fly north, to Britain and Ireland.

Researchers studying the birds discovered that males and females who had wintered in the same place were more likely to mate with each other during the summer when all the birds were back in central Europe. They also found that birds wintering further north also produced larger clutches and more young. The researchers speculated that this happens for two reasons. First, males who winter in the north have to travel a shorter distance when returning to their summer breeding grounds than those who wintered in Spain or Africa. Therefore, these males stake out the territories that have the best food resources before their southern counterparts arrive. Another reason may be that females who return to the breeding ground after wintering in the north are less exhausted and have more resources to devote to raising more young.

The researchers think the finding could represent one way that two species gradually form from one. In time, the two groups of blackcaps—those that winter in the north versus those that winter in the south—may no longer breed with one another and become separate species.

The finding was detailed in a October 20^th issue of the journal Science. 

--Ker Than

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Credit: Dr. Wolfgang Fiedler, Vogelwarte Radolfzell/Max Planck Society