Behavioral and genetic tests of reproductive isolation among brood parasitic indigobird species and host races

Indigobirds (genus Vidua) are host-specific brood parasites that have diversified in a recent radiation driven by host shifts. Imprinting on their estrildid finch host species, female indigobirds choose mates based on their mimicry of host song and parasitize nests of the species that reared them. Thus, song learning and mimicry result in reproductive isolation of indigobird populations associated with different hosts and promote speciation following host colonization. These same behavioral mechanisms, however, may also lead to hybridization among established species and it is not clear to what extent the limited genetic differentiation among indigobird species is due to retained ancestral polymorphism versus ongoing hybridization.; At Tibati, Cameroon, three morphologically distinct indigobird species parasitize four different host species, making it an ideal location to test for reproductive isolation or, alternatively, gene flow between species. In playback experiments, male indigobirds responded more aggressively to conspecific than heterospecific songs, suggesting that heterospecific mating between indigobirds is infrequent. Even if mating is strongly determined by learned song preferences, however, hybridization may result if females occasionally parasitize the hosts of other indigobird species. If so, close relatives should occasionally be found singing the songs of different hosts. Supporting this prediction, genetic parentage analyses identified a small number of putative father-son and sibling pairs that sang different mimicry songs. It is therefore likely that infrequent host choice errors result in gene flow and contribute to the genetic similarity of indigobird species. Results from multi-locus population genetic analyses support this conclusion, but morphological analyses show no evidence of hybridization.; Ectoparasitic lice on indigobirds and their avian hosts were examined to test the hypothesis that indigobirds carry louse species characteristic of their respective hosts. A molecular phylogeny of louse lineages, however, revealed that indigobirds and their hosts carry distinctive ectoparasite faunas. This result challenges the conventional paradigm that lice are transmitted predominantly at the nest, and suggests that coevolution between louse and avian lineages may be more important than the ecology of louse transmission in determining the specific associations of lice.