The location of ranging cues in wren song: Evidence from calibrated interactive playback experiments

Many species of songbirds appear to monitor the distance to conspecifics by attending to songs received from them (ranging). Features of the habitat-induced degradation of the song seem to provide the ranging cues. This study investigates where, in a received, degraded song, the cues used for ranging lie; in the song elements, in the inter-element pauses or both. Examples of undegraded and degraded wren Troglodytes troglodytes song were cut into element and pause portions which were reassembled into four treatment types: two where the degradation between elements and pauses was concordant and two where either elements or pauses were degraded. These were played back at the boundaries of wren territories. The experiment was calibrated by measuring the degradation of the received playback songs and re-categorizing the trials into treatment types accordingly. Wrens responded to undegraded song by aggressively approaching and to degraded song with a less intense approach and an increase in perch height. When the level of degradation within the different song portions was concordant, subjects' responses were as expected from a wren ranging an intruder (undegraded) or a distant conspecific (degraded). When only the pauses were derived from a degraded song the behaviour closely resembled a wren ranging a distant rival, however the signal with only degraded elements elicited a response of intermediate intensity. We conclude that wrens seem able to adjust their response relative to degradation cues in either the elements or the pauses of degraded songs. In addition, the pause degradation seems to offer cues that can be used independently of element degradation, whereas element degradation needs the congruity of cues within the pauses to evoke a response equivalent to that elicited by the degraded song.