What birds eat may determine who they mate with, according to a new study of Darwin's finches. Finches with sturdy bills built for crushing tough seeds can't trill as complex a song as their fellows whose slender little beaks are superb for snatching insects. And if the species don't recognize each others' songs, they won't interbreed. The finding suggests that adaptation to different food types, by altering finches' songs, helps establish new species.
In the traditional scenario for speciation, a population of animals becomes isolated from others of its kind, say by being blown onto a far-off desert island like one of the Galápagos. Over generations, the castaways eventually change so much that, even if reunited, they wouldn't recognize animals from the mainland as potential mates.
Such divisions may also occur by adaptation to new foods, suggests behavioral ecologist Jeffrey Podos of the University of Massachusetts in Amherst. If beak shape changes, he reasoned, it might mean that the bird can't sing the same old song--just as a flute and a tuba sound different. So in 1999, he tagged a variety of Galápagos finches with color bands and recorded each bird's song. Heavy-billed species like the large ground finch didn't sing the same range of frequencies or trill as fast as small-beaked species like the warbler finch. The same was true for the medium ground finch, a species with lots of variation in bill shape. Birds with basso profundo beaks didn't turn in as dazzling a performance, he reports in the 11 January issue of Nature.
These changes, by affecting how animals identify potential mates, are key to the creation of new species, says animal behaviorist Michael Ryan of the University of Texas, Austin. Podos's study "is one of the few, if not the only," example showing that adapting to a new niche automatically changes an animal's mating signal, Ryan says.