A population of songbirds that shares summer breeding grounds may be dividing in two based on where they spend their winters, according to a report in this week's Science. European blackcaps that follow a new migratory path tend to mate with each other and reproduce more successfully than birds that stick to the traditional route. This split into two breeding groups could increase the likelihood of the birds forming separate species, the authors note.

This is the first time biologists have directly observed a mechanism -- selective mate choice -- that is separating a species into two populations, said Pete Marra, a senior scientist at the Migratory Bird Center of the National Zoo in Washington, DC. "To actually have this demonstrated is nice," noted Marra, who was not involved in the study.

Blackcaps have long been summer visitors to the British Isles, but in the early 1960's, birders...

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