New migration, new species

Study suggests songbirds are choosing mates that share a new migration path, splitting one group into two

By | October 21, 2005

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 began to notice them in winter, as well. In recent years, the number of wintering blackcaps has swelled – backyard birdwatchers in Britain reported seeing them in 31% of their gardens between October 2003 and March 2004, according to a survey conducted by the British Trust for Ornithology.

Biologists have previously traced this burgeoning population to their breeding grounds in southern Germany and Austria, where the majority of blackcaps travel southwest to the Iberian peninsula and North Africa in the off season. When a growing number of birds began migrating northwest, however, Stuart Bearhop of Queen's University Belfast and his colleagues set out to look for the forces driving this shift in behavior.

The birds all look alike and mingle in their summer habitat, according to the authors. To determine where individual birds spent the winter, the scientists measured traces of hydrogen isotopes that had naturally accumulated in the birds' toenails. Specifically, they looked at the ratio of deuterium to hydrogen, which varies regionally and is lower in England's rainwater than in Spain's. As a result, birds found in the UK during winter have less deuterium in their claws than those found in Iberia, about a thousand miles to the south.

The scientists caught blackcaps soon after they arrived in central Europe and analyzed a small sliver of the end of the birds' toenails, which formed before the spring migration began. "There's no metabolism in claws," Bearhop said, so their isotopic composition reflected that of the water they drank when they formed. "It preserves a little record of where the birds have been," he added.

When the team sorted the birds at the summer breeding grounds using the isotopic signature to identify their winter habitat, they found blackcaps were more likely to pair up with a mate that wintered in the same place. The researchers also found that northern birds bred more successfully -- laid more eggs and were more likely to fledge a chick -- than ones from the south. Males from the British Isles arrived a few days earlier than their southern rivals, a head start that may enable them to claim a better territory, Bearhop suggested.

Blackcaps inherit a preference for migratory direction, according to previous work by co-author Peter Berthold of the Max Planck Institute for Ornithology in Radolfzell, Germany. That combined with the relative success of breeding pairs from the north means that ever more birds will likely be following the new migratory path, the authors note.

"We don't know much about the evolution of migration," said Frank Moore of the University of Southern Mississippi in Hattiesburg, who also studies bird migration but was not involved in the research. This study is exciting, he added, because it "provides insight into the evolution of this alternative migratory path."

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