Social evolution and vocal evolution may be strongly linked. The starlings are songbirds known for their complex song repertoires and learning abilities. In the Red-winged Starling (Onychognathus morio), variation in social organisation (territorial to more colonial breeding) allows an assessment of the influence of social factors on vocal communication within a single species. We compared the whistle characteristics among three populations with different breeding associations in the Eastern Cape, South Africa. During one breeding season, we recorded five widely-separated (>1 km apart) territorial pairs breeding in isolated mountains, eight territorial pairs at higher density on a university campus (<100 m apart) and two dense colonies on cliffs with nests <10 m apart. Our results show that each isolated territorial pair had a large vocal repertoire and produced its own vocal themes. Conversely, as the number of social partners increased, vocal sharing increased and the repertoire size was reduced. The influence of habitat type and genetic differences between populations on vocal variations was also investigated; the presence of more social partners seemed to be the key factor inducing greater uniformity in vocalisations. In social groups, individuals in regular contact need to use the same channel of communication to regulate their interactions. There are interesting parallels with the evolution of language.