1. Department of Biology, Boston University, Boston, Massachusetts 02215, USA
2. Museum of Zoology, University of Michigan, Ann Arbor, Michigan 48109, USA
3. Department of Ecology and Evolutionary Biology, University of Michigan, Ann Arbor, Michigan 48109, USA

Correspondence to: Michael D. Sorenson^1,2 Email: msoren@bu.edu
DNA sequence data have been deposited in GenBank under accession numbers AF090341, AF407021-AF407036, AF407107-AF407122, AY136611, AY322613-AY322837, AY323530-AY323607 and AY324232-AY324306.

Abstract

A growing body of empirical and theoretical work supports the plausibility of sympatric speciation^{1, 2, 3}, but there remain few examples in which all the essential components of the process are well understood. The African indigobirds Vidua spp. are host-specific brood parasites. Indigobird nestlings are reared along with host young, and mimic the mouth markings of their respective hosts^{4, 5, 6}. As adults, male indigobirds mimic host song^{4, 5, 6, 7}, whereas females use these songs to choose both their mates and the nests they parasitize⁸. These behavioural mechanisms promote the cohesion of indigobird populations associated with a given host species, and provide a mechanism for reproductive isolation after a new host is colonized. Here we show that all indigobird species are similar genetically, but are significantly differentiated in both mitochondrial haplotype and nuclear allele frequencies. These data support a model of recent sympatric speciation. In contrast to the cuckoo Cuculus canorus, in which only female lineages are faithful to specific hosts^{9, 10}, host switches have led to speciation in indigobirds because both males and females imprint on their hosts^{8, 11}.