Responses to song playback vary with the vocal performance of both signal senders and receivers

Abstract

Mating signals of many animal species are difficult to produce and thus should indicate signaler quality. Growing evidence suggests that receivers modulate their behaviour in response to signals with varying performance levels, although little is known about if and how responses are affected by receiver attributes. To explore this topic we conducted two experiments with swamp sparrows, Melospiza georgiana, in which we challenged territorial males with playback of songs with trill rates that were natural, digitally reduced, or digitally elevated (control-, low-and highperformance stimuli, respectively). In our first experiment, we found that males responded more aggressively to control songs than to low-performance stimuli, that low-performance stimuli with the most severe trill-rate reductions elicited the weakest aggressive responses, and that the subjects' own trill rates predicted aggressive responses. In our second experiment, we found that male responses to high-performance stimuli varied significantly, in ways predicted by two factors: the degree to which we had elevated stimulus performance levels of high-performance stimuli, and subjects' own vocal performance levels. Specifically, males were less aggressive towards stimuli for which we had elevated performance levels to higher degrees, and subject males with higher vocal performances themselves responded more aggressively. These findings together offer a novel illustration of how responses to aggressive signals may rely not just on signal attributes, but also on attributes of responding animals themselves.

Anthropogenic noise weakens territorial response to intruder's songs

“…The use of average songs was essential for two reasons. First, behavioral responses to song can change in response to differences in a variety of song features (Brumm and Ritschard 2011, Halfwerk et al 2011, Moseley et al 2013. Second, the two species included here are known to shift the minimum frequency of their songs in response to background noise (~500 and 400 Hz, respectively, for spotted towhee and chipping sparrow; Francis et al 2012b, unpublished data) and our collection of local songs suitable for playback experiments reflected this variation.…”

“…See Table 3 for further description of these variables. We measured these variables, which include both movement and song production, because they are representative of typical responses to competitors (Moseley et al 2013). Immediately after completion of the playback trial, we measured A-weighted background noise amplitudes (equivalent continuous noise levels [L eq ], fast response) for 1 min with NIST-certified sound pressure meters (Casella CEL320/CEL1002 converter) from the bird's original perch location at the beginning of the trial.…”

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

“…In particular, to the extent that trill rate serves as a reliable indicator of territorial threat [9][10][11], chipping sparrows with a low trill rate neighbour should benefit by retaining that neighbour as a 'dear enemy' [12], in favour of a new neighbour with a higher trill rate. In contrast, chipping sparrows should have little incentive to assist neighbours who themselves have a faster trill rate, and especially not when that neighbour is challenged with an even faster intruder: indeed, we never observed coalitions forming under such circumstances.…”

“…Males that can best execute challenging motor displays tend to be treated as superior rivals [8], and in songbirds accumulating evidence suggests that high-performance trills, e.g. fast trills, are especially threatening [9,10].…”

“…We used a repeated measures design and Wilcoxon signed-ranks tests to compare receiver response to fast versus slow trill rate, for univariate responses as well as combined behavioural responses ( principal component analysis (PCA) scores). We also asked whether subjects' responses to playback covaried with the degree to which their own trill rates differed from stimulus trill rates [10]. Finally, for trials in which coalitions formed, we compared trill rates of residents and allies and asked whether particular triadic relationships were more conducive to coalition formation than others.…”