Quiet violence: Savannah Sparrows respond to playback‐simulated rivals using low‐amplitude songs as aggressive signals

Abstract

When animals compete over resources such as breeding territories, they often use signals to communicate their aggressive intentions. By studying which signals are associated with aggressive interactions, we gain a deeper appreciation of animal behaviour. We studied aggressive signalling in male Savannah Sparrows, Passerculus sandwichensis, focusing on signals that precede physical attack against territorial intruders. We simulated intruders using song playback and taxidermic models, and we determined which behaviours were associated with physical attack. Previous studies that have used this approach suggest that many species produce songs of dramatically lower amplitude, or “soft songs,” as signals of aggressive intention. Savannah Sparrows, however, are not known to produce soft songs, and therefore, they provided an interesting system for testing signals that predict attack. Of 93 playback subjects, 23 males attacked the simulated intruder and 70 did not. To our surprise, Savannah Sparrows produced soft songs, and the number of soft songs was a significant predictor of attack on the simulated intruder. Birds also showed a nonsignificant tendency to produce more “chip” calls prior to attack on the simulated intruder, whereas three other measured behaviours (aggressive calls, wing waving and passes over the model) did not predict attack. Our study contributes to the growing body of research on aggressive territorial signals and reveals that soft song is an even more widespread signal of aggression in songbirds than previously recognized.

Soft song provokes stronger aggressive responses than broadcast song in the Plain Laughingthrush (Pterorhinus davidi)

“…We found that playback of male soft song provoked a significantly stronger aggressive intent response than broadcast songs in the Plain Laughingthrush, perhaps because the responder perceives the intruder to be closer, as the signal was played at a much lower amplitude. This finding is consistent with other studies of other passerines including Eurasian Blackbird Turdus merula (Dabelsteen & Pedersen 1990), Brownish-flanked Bush Warbler Horornis fortipes (Xia et al 2013), Savannah Sparrow Passerculus sandwichensis (Moran et al 2018), Dark-eyed Junco Junco hyemalis (Reichard et al 2011) and Song Sparrow Melospiza melodia (Anderson et al 2012, Templeton et al 2012. However, in the Ortolan Bunting Emberiza hortulana, soft song is used by males in an aggressive context, although it does not appear to represent a true aggressive signal (Jakubowska & Osiejuk 2018).…”

“…2013), Savannah Sparrow Passerculus sandwichensis (Moran et al . 2018), Dark‐eyed Junco Junco hyemalis (Reichard et al . 2011) and Song Sparrow Melospiza melodia (Anderson et al .…”

“…Soft songs are employed by individuals during aggressive encounters to conceal themselves from rivals and to minimize information transfer about the aggressive interaction with the rival to eavesdroppers (Dabelsteen & Pedersen 1990, Ręk & Osiejuk 2011, Moran et al . 2018). For soft songs to serve as a reliable signalling strategy for aggressive interactions (e.g.…”

“…In other instances, soft songs are also produced in the same social contexts as broadcast songs (Reichard & Anderson 2015), and for some passerines they represent a significant predictor of stronger aggressive responses than broadcast songs (e.g. Dabelsteen & Pedersen 1990, Anderson et al 2007, Xia et al 2013, Moran et al 2018.…”

Cumulative cultural evolution and mechanisms for cultural selection in wild bird songs

“…Stimuli were presented in a randomized order, and each stimulus set was used at least twice. The response duration and behaviours of males (crouching with head feathers flattened close to the skull, aggressive displays 48 and vocalizations 90 ) were noted. We used duration, measured as time from the end of the stimulus presentation until the male ceased responding (defined as moving 20 m or more away from the speaker, or singing a full and loud song, or engaging in feeding or preening behaviour), as our primary measure of male response 55 .…”