Team of rivals: alliance formation in territorial songbirds is predicted by vocal signal structure

Abstract

Cooperation and conflict are regarded as diametric extremes of animal social behaviour, yet the two may intersect under rare circumstances. We here report that territorial competitors in a common North American songbird species, the chipping sparrow (Spizella passerina), sometimes form temporary coalitions in the presence of simulated territorial intruders. Moreover, analysis of birds' vocal mating signals (songs) reveals that coalitions occur nearly exclusively under specific triadic relationships, in which vocal performances of allies and simulated intruders exceed those of residents. Our results provide the first evidence that animals like chipping sparrows rely on precise assessments of mating signal features, as well as relative comparisons of signal properties among multiple animals in communication networks, when deciding when and with whom to form temporary alliances against a backdrop of competition and rivalry.

Reply to Akçay & Beecher: yes, team of rivals in chipping sparrows

“…Yet the raw parameters themselves, trill rate and frequency bandwidth, are also proper indices of vocal performance because, in general, faster or wider bandwidth trills are harder to produce [3]. Our demonstration in chipping sparrow songs of a trade-off between maximal trill rate and frequency bandwidth [1] suggests that any of these parameters might signal vocal performance. Yet determining which are salient during vocal communication requires controlled perceptual tests that isolate the effect of each parameter, and variation therein, on birds' responses [1].…”

“…The main conclusions of our original report [1] were that male chipping sparrows form defensive coalitions in response to simulated territorial intrusion, and that coalition formation is predicted by relative structural properties of birds' songs. Akçay & Beecher (hereafter 'A&B' [2]) critique our report on a number of fronts including study design, methods, analysis and interpretation.…”

“…Our demonstration in chipping sparrow songs of a trade-off between maximal trill rate and frequency bandwidth [1] suggests that any of these parameters might signal vocal performance. Yet determining which are salient during vocal communication requires controlled perceptual tests that isolate the effect of each parameter, and variation therein, on birds' responses [1]. We now know that chipping sparrow males attend to trill rate, as birds' responses to playback in our original study ( [1], non-coalition trials) covaried with trill rates of both stimuli and subjects.…”

“…Yet determining which are salient during vocal communication requires controlled perceptual tests that isolate the effect of each parameter, and variation therein, on birds' responses [1]. We now know that chipping sparrow males attend to trill rate, as birds' responses to playback in our original study ( [1], non-coalition trials) covaried with trill rates of both stimuli and subjects. By contrast, it is unknown whether chipping sparrows perceive or attend to variations in frequency bandwidth or thus, by extension, vocal deviation.…”

Team of rivals in chipping sparrows? A comment on Goodwin & Podos

“…Goodwin & Podos [1] (G&P hereafter) report an interesting study that claims to show male chipping sparrows form tactical defensive coalitions with their neighbours to fend off intruders. The most novel claim in the paper is that would-be allies make decisions to help or not depending on the relative trill rate of their neighbour's, the intruder's and their own song.…”