Egg-deploying bird species divide for a song.

Musical taste, rather than geography, may have split Africa's indigobirds into multiple species, and a new analysis gives a genetic underpinning for that idea.

This scenario puts indigobirds among the few vertebrates for which scientists have strong evidence that species divided without some geographic barrier looming, says Michael D. Sorenson of Boston University. His colleague Robert B. Payne of the University of Michigan in Ann Arbor proposed this notion years ago, but genetic evidence now provides critical support, say Sorenson, Payne, and Kristina M. Sefc of Boston University in the Aug. 21 Nature.

"This paper does a good job of nailing that down," says Stephen Rothstein of the University of California, Santa Barbara.

The standard scenarios for creating species raise a mountain range, loose a river, or provide some other geographic barrier that severs contact between parts of a population. As time goes by, the groups on each side of the obstruction adapt to their settings or randomly drift apart, eventually growing so different that if they meet again, they don't mate. Recently, though, evolutionary biologists have been looking for examples of species that diverged with no geographical boost (SN: 7/21/01, p. 42). Called sympatric or sometimes ecological speciation, these splits happen despite overlapping ranges.

The evidence for such events looks strong among insects, says Sorenson, but vertebrate splits have been trickier to find.

Indigobirds, which belong to the genus Vidua, lay their eggs in other birds' nests, targeting the hospitality of particular species. A study in 1964 proposed that when a host species such as a firefinch splits, the corresponding indigobird species splits, too.

Payne, however, has argued against that idea. In a 2001 study of captive village indigobirds from Africa, he and his colleagues fostered nestlings with two host species: the red-billed firefinch, which these indigobird parents would target in the wild, and the Bengalese finch, which these indigobirds wouldn't naturally encounter.

As they do in the wild, the young male indigobirds picked up the songs of their foster parents. When the fostered indigobirds grew up, females preferred males with songs like those the females had heard in their foster nests, even if the nest belonged to the unusual host species.

Here, Payne argued, speciation was just waiting to happen. Indigobirds that grew up in a novel nest could accept the songs of their new foster parents and prefer mates with the same oddball background.

A small genetic analysis 5 years ago found some support for the idea that indigobirds diverged from other birds much more recently than their hosts did, a finding that favors the song scenario over cospeciation of indigobirds and hosts. The new, more extensive study comes to the same conclusion. It examined 200 indigobirds from 10 African species, plus their host species.

The host species, various firefinches, started diverging from one another 7 million years ago. But according to the new data, the parasitic birds didn't diversify until 500,000 years ago or even more recently, says Sorenson. This dashes the idea of the hosts and parasites evolving in lockstep.

Differences in gene frequencies made it clear that the species of indigobirds are distinct, even though the entire group is unusually similar genetically, Sorenson says.