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Why birds change their tune

B. Rosemary Grant / Science via AP file

A species of ground finch (Geospiza fortis) on Daphne Major in the Galapagos Islands appears to have sped up the trill of its tune to differentiate itself from a similar species that colonized the island in 1983, researchers say.

More than a dozen species of birds in the Galapagos Islands served as prime experimental subjects for Charles Darwin's ideas about evolution -- and today, "Darwin's finches" still provide examples of evolution at work. The latest example, revealed today, suggests that the songbirds modify their tunes to distinguish themselves from similar species.

The finches in question are Geospiza fortis (medium ground finch) and G. scandens (cactus ground finch), longtime residents of Daphne Major Island in the Galapagos chain. The males of each species have a song that's characteristic enough to ensure that the females of the species respond to the right mating call. There might be individual variations that crop up as each father teaches his sons to sing -- but the features of the song, such as the trill and the tempo, has generally stayed close to the norm.

Until 1983, that is.

That's when another species, the large ground finch (G. magnirostris), moved onto Daphne Major and began growing in numbers. This third species had songs that were somewhat similar to that of the other two species. But as time went on, G. fortis and G. scandens changed their typical songs: The trills became faster, while the duration of notes and the inverval between them became shorter.


All these changes were "in the direction away from G. magnirostris in acoustical space," Princeton's B. Rosemary Grant and Peter Grant, husband-and-wife ornithologists who have been studying Darwin's finches for decades, report in this week's issue of the Proceedings of the National Academy of Sciences.

These observations fit the Grants' hypothesis that songbirds tend to change their tune if there's a "fitness penalty" for interbreeding with a species that sings a similar song. Researchers call this learning phenomenon a "peak shift."

"Songs of the residents and colonists may gradually change over the next few generations under pressures for unambiguous transmission to other members of the same breeding population, both from the habitat and from other species," the Grants wrote.

During the same period, finch species in the Galapagos have undergone other changes as well. For example, the beaks of G. fortis gradually became smaller, because that enabled the birds to shift to a diet of smaller seeds and avoid competition with the bigger birds that moved in. But the researchers determined that the songbird shift was a separate phenomenon.

The Grants emphasize that their study "is purely observational, without experimental control of potentially confounding variables, and hence our identification of causes should be considered as an hypothesis rather than a demonstration." But if the hypothesis holds up, the changes in tune could demonstrate how two populations that have been separated and are starting to differentiate use behavioral signals to build a wall between themselves -- thus fostering the origin of new species. What's more, they suggest that speciation can occur very quickly when driven by learned behavior.

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The Grants' research paper, "Songs of Darwin's Finches Diverge When a New Species Enters the Community: Implications for Speciation," is being published in the Proceedings of the National Academy of Sciences as part of a series of Inaugural Articles by academy members elected in 2008. To learn more about the Grants and Darwin's finches, check out "The Beak of the Finch: A Story of Evolution in Our Time" by Jonathan Weiner, or the Grants' own book, "How and Why Species Multiply." Weiner's Pulitzer-winning book serves as the latest selection for the Cosmic Log Used Book Club, which highlights books with cosmic themes that have been around enough to be available at your local library or through secondhand-book shops.

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