Hybrid vocalizations are effective within, but not outside, an avian hybrid zone

Secondary contact between closely related species can lead to hybridization. The fitness of hybrid individuals within and outside the hybrid zone determines whether the hybrid zone expands into the ranges of the 2 parental species or remains a stable, geographically narrow area in between the allopatric ranges of the parental species. In birds, vocalizations play an important role in male–male competition and female mate choice and are often affected by hybridization. One of the factors that will influence male hybrid fitness is the ability to defend a territory against competitors by vocalizing. We tested the efficacy of territorial signals of hybrids of 2 dove species, Streptopelia vinacea and Streptopelia capicola, compared with the vocalizations of the parental species. With playback experiments, we assessed the response to hybrid and the 2 parental species vocalizations in the hybrid zone and adjacent allopatric populations of each species. In the hybrid zone, males did not respond differently to the 3 vocalization types. In both allopatric populations, however, males responded more to conspecific than to heterospecific signals and the response strength to hybrid signals was intermediate. Therefore, in the allopatric populations, hybrid males may have a reduced success in defending territories. In male–male interactions in the hybrid zone, hybrids may not have a disadvantage compared with males of the parental species. The ability to defend a territory against competitors may thus help maintain a stable hybrid zone in the area of overlap.