Breeding density affects the honesty of bird vocal displays as possible indicators of male/territory quality

Vocal displays are supposed to be an honest signal of the phenotypic and genetic quality of individuals and their territory. Moreover, signal interactions are nearly always associated with individuals in aggregations, and their function could in part be explained as social behaviour. Conspecific density has been shown to be a particularly strong proximate and ultimate factor acting on several individual/population features; thus, it may be expected to affect vocal behaviour too. Here, I investigate the hypothesis that, in long-lived, territorial species, density affects the vocal displays of mated males, masking their honesty as a possible signal of male/territory quality. Each month I listened to the dusk calls of 17 breeding male Eurasian Eagle Owls Bubo bubo during their prelaying period. Nine males bred in a low-density situation, the other eight in a high-density one. Conspecific density was found to affect the honesty of call features as signals of male and/or territory quality. The call display as a reliable predictor of male fitness measured as productivity persisted only in situations of high breeding owl density, where male-male competition was stronger. Accommodation of call activity allows individuals to minimize the costs of aggressive calling by adjusting the territoriality threshold to local conditions. The results of this study emphasize the importance, when investigating the evolution and maintenance of honest territorial or sexual signals, of considering the environmental and social context experienced by the individual, thereby corroborating the idea that male-male competition contributes to the maintenance of honest signalling.