Song discrimination suggests premating isolation among sympatric indigobird species and host races

Indigobirds (Vidua spp.) are host-specific brood parasites thathave diversified in a recent radiation apparently driven byhost colonization. Behavioral imprinting of both male and femaleindigobirds on host song is thought to promote rapid speciationbecause it results in assortative mating between indigobirdsassociated with a particular host. We conducted a song playbackexperiment to test whether male indigobirds discriminate amongpotential competitors based on song. Of particular interestwas the behavior of two sympatric host races of the Cameroonindigobird Vidua camerunensis that differ only in host songmimicry and other components of their vocal repertoires. Territorialmales of the two V. camerunensis host races and Vidua raricola,a morphologically distinct indigobird species, were tested withplaybacks of each other's songs. Males of all three groups respondedmost aggressively to songs of their own species and/or hostrace, as evidenced by strong and highly significant differencesin a variety of response variables. This differential territorialdefense suggests that an intruding male with different songsdoes not represent a competitive threat and is expected if femalesmate assortatively with respect to song. Thus, our results provideevidence of premating reproductive isolation among recentlyevolved indigobird species and host races.