Adopted melodies are the driving force behind indigobird evolution, according to new research. The strange discovery demonstrates how new species can emerge without first being separated by geographical barriers.
Like cuckoos, African indigobirds lay their eggs in the nests of other birds. Ornithologist Robert Payne of the University of Michigan, Ann Arbor, had shown that young indigobirds develop an affinity for the song of their adopted parents: Males grow up to croon melodies cribbed from their host fathers, while females listen for the same song to locate mates. The preference seems to be learned, Payne found. When he moved indigobird eggs into the nests of unfamiliar hosts, the chicks picked up those songs and parasitized the nests of those new hosts when they grew up.
Now Payne and Michael Sorenson of Boston University suggest that this habit leads to new indigobird species. They hypothesized that females' preference for males with familiar songs would keep them from interbreeding with birds singing different songs. That in turn would allow genetic differences to accumulate between groups with different song preferences and eventually lead to the creation of new species. In the 21 August issue of Nature, the pair reports genetic studies that support this view. While the genetic stock of the entire clan of indigobirds is remarkably uniform, certain gene sequences pop up more often in some species than others. "This suggests that interbreeding is quite rare," Sorenson says. The findings also suggest that new species can be spun off quickly--possibly in as little as one generation.
So why are the females so choosy? The researchers think it has to do with a quirk of finch biology. The chicks of firefinches, the indigobirds' hosts, bear a characteristic pattern of mouth spots that fades with maturity. The spots may help parent finches recognize their chicks. But indigobird chicks mimic the mouth spot pattern of their hosts. An indigobird hen may use song to find a mate with genes that will produce convincing disguises for chicks, says Payne. "She knows he grew up in the same kind of nest, so together they can probably make babies that can fit into that kind of nest."
Scientists call the research a convincing example of how species can develop within overlapping ranges--a process known as sympatric speciation. "I think it's probably one of the best, and only, examples of sympatric speciation in birds," says Robert Zink of the University of Minnesota, Twin Cities. "It also shows how relatively complicated it is for this type of speciation to occur. No wonder this route is so rare in birds."