Simulated courtship interactions elicit neighbour intrusions in the whitethroat, Sylvia communis

Animals often signal in communication network environments that include nonintended receivers that may constitute a risk to the signalling animals. Unpaired male songbirds, which sometimes intrude into neighbouring territories and interrupt courtship, may be an example of nonintended receivers of vocal courtship signals. We investigated this in the whitethroat, using dyads of unmated territorial males. Each dyad consisted of a resident male, who was part of the experimental treatment, and a neighbouring male, whose responses to the treatment stimuli were tested. Each dyad received two playback treatments: a courtship treatment in which the treatment male was induced to court a remotely controlled calling and jumping stuffed female, and a song duel treatment in which playbacks of male song were used to induce the focal male to countersing. The neighbours responded with more song flights to the courtship treatment than to the song duel treatment, perhaps to achieve an overview of the courtship activity or attract the female. Only the courtship treatment resulted in neighbouring intrusions and immediate chasing of the intruder by the treatment male. The males that suffered intrusions did not sing less than those that did not suffer intrusions. Reduced singing therefore may not be able to explain the intrusions. Every intrusion except one occurred after the first courtship vocalization, making it likely that the courtship per se caused the intrusions. This emphasizes the necessity of restricting the transmission range of courtship vocalizations in a network and therefore also explains why the courtship vocalizations of whitethroat males are relatively short ranging.