On Evolution’s Hit Parade, Indigobirds Sing Songs That Drive Species Formation

in College and Graduate School of Arts and Sciences, News Releases, Science & Technology
August 29th, 2003

Contact: Ann Marie Menting, 617/353-2240 | amenting@bu.edu

(Boston, Mass.) – Indigobirds may never win “parent of the year” awards. Known as brood parasites, they are more the “park-and-fly” type; a female lays an egg in the nest of another bird species and then wings away, confident that her egg—and the hatchling that develops—will be cared for by the nest owners.

Although this behavior may not make indigobirds model parents, it does help make them models of evolutionary change for scientists who study the birds’ behavioral and physical adaptations for clues to how species form.

“Indigobirds offer us a textbook case of evolution in action,” says evolutionary biologist Michael Sorenson, an assistant professor of biology at Boston University. “These birds provide one of the few models of a system in which speciation occurs without geographic isolation.”

Theorists have debated how new species form since the ink began to dry on Charles Darwin’s The Origin of Species. Overall, it is thought that speciation occurs over long periods of time as a result of interbreeding among members of a group or population that are, for one reason or another, isolated from other members of the same population.

Sorenson sees indigobirds as a model for what is known as sympatric speciation. Moreover, he and his research colleagues have genetic evidence that points to a behavioral trait—the songs the birds use to attract mates—as the driving force behind their speciation. Their findings appear in the August 21st issue of Nature.

In traditional theory, geographical barriers are the isolating mechanism. If, for example, geologic activity changed a land area so that mountains rose and split a region populated by a species of birds, the bird groups on either side of the mountain would no longer be able to breed together. Their genetic information, complete with changes that lead to physical or behavioral adaptations to the demands of their environments, would no longer be pooled and passed along to new generations. Eventually, the birds in the two groups would become sufficiently different to qualify as separate species.

The theory of sympatric speciation attempts to explain speciation that occurs in the absence of geographical barriers. In this theory, geographic barriers are supplanted by some other isolating mechanism. For indigobirds, a process known as behavioral imprinting provides the impetus for speciation. This imprinting starts in the nest.

Found in Africa and organized into approximately 10 species, indigobirds are brood parasites that are host specific. That is, any given species of indigobird consistently chooses to abandon its eggs in the nests of a particular bird species. For indigobirds, the hosts-of-choice are firefinches. The village indigobird species, for example, usually deposits eggs in the nests of the red-billed firefinch.

Research shows that this consistent choice of adoptive parents has selected for certain traits in indigobird offspring. Indigobird mouth markings, a pattern of markings and color most visible to firefinch parents feeding open-mouthed hatchlings, mimic those of their preferred finch species.

Young indigobirds also mimic the song of the finches they grow up with. A male village indigobird will sing the songs of the red-billed firefinch, while females of the species will choose as mates only those males that sing the song of the red-billed firefinch. Female village indigobirds also use the song to locate red-billed firefinch nests in which to deposit their next generation.

This strong reliance on host-parent behavior—behavioral imprinting—has led to the reproductive isolation needed for speciation, Sorenson and colleagues argue. To study this, they looked at the mitochrondrial DNA (mtDNA) of various indigobird species. Each mtDNA sequence is called a haplotype and, except for the occasional mutation, is passed intact from mother to offspring. By examining these mutational changes and the distribution of specific haplotypes of different indigobird species, the researchers were able to piece together the evolutionary history of indigobird species.

Their results show that the common ancestor of all indigobird species lived less than a million years ago and that current species of indigobirds likely evolved within the past few tens of thousands of years, a mere sliver in evolutionary time. More importantly, the new genetic evidence indicates that indigobird species, although genetically very similar, maintain genetic differences by the songs they sing.

Just as a mountain would prevent birds on either side from interbreeding, the adopted songs of indigobirds dictate mate choice, producing reproductively isolated populations that drive their speciation.

Genetic data also point to occasional mixing or hybridization among indigobirds. According to the researchers, these genetic hybrids result from egg-laying “mistakes,” when a parent indigobird inadvertently deposits an egg in the nest of a firefinch species that is not its traditional host. Raised by the “new” firefinch, the indigobird young adopt that bird’s song, a tune that makes them an attractive mate to a different species of indigobird, one that uses that firefinch as its brood host. The origin of a new indigobird species, therefore, may occur when eggs are laid in the nest of an entirely new host species.

Note to editors: Sorenson’s paper, “Speciation by host switch in brood parasitic indigobirds,” can be accessed at Sorenson Nature 082103*. Images and songs of certain indigobirds and finches are available at http://people.bu.edu/msoren/indigos.html.

* This file can be viewed using Adobe Acrobat. For a free version of Acrobat Reader, please visit Adobe Systems, Inc. (http://www.adobe.com/products/acrobat).

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