The duets of birds have intrigued biologists for a long time, yet much remains unknown about the evolution of these striking collective displays. This is because previous studies on duet evolution have been biased to songbirds and neglected other bird groups. In songbirds, the absence of migration has been found to predict the occurrence of duetting, indirectly supporting the idea that duet communication is linked with pair-bonding. Here, we used phylogenetic comparative analyses in a sedentary clade of non-songbirds, the barbets (Capitonidae), to reveal new correlates of duet evolution in birds. We found (1) that duets evolved several times independently in different barbet lineages and (2) that duetting evolved in association with group living (i.e. the presence of helpers or non-breeding adults during the breeding period), but not with sexual monochromatism or habitat type. We conclude that the evolution of social systems has a major influence on the evolution of collective signalling such as duetting. When social living causes resource competition among group members or increases the risk of paternity loss, duetting by primary pairs would play a key role in signalling mate guarding and dominance against subordinate group members.

We collected information on the presence/absence of duets for each species of the family Capitonidae from pubished literatures, and the web-based sound library Xeno-Canto (http://www.xeno-canto.org). Informatin on body size, habitat, and sociality was also collected based on published literatures.