Sing softly and carry a big stick: signals of aggressive intent in the song sparrow

Reliability of signalling has been the focus of much research in behavioural ecology, yet few studies have been able to show that putative aggressive signals actually predict subsequent escalation (e.g. attack). This is partly because until recently researchers have usually used stimuli (e.g. song playback) that did not permit subsequent escalation in the form of attack. We presented male song sparrows, Melospiza melodia, with a traditional simulated intrusion that involved only playback of their own song (no-mount condition), or a simulated intrusion that coupled playback of their own song with a taxidermic mount. We hypothesized that aggressive signals that are predictive of attack should increase in frequency when there is a visible intruder present (i.e. in the mount trials). Our results showed that only low-amplitude song (soft song) and wing waves (the latter effect only approached significance) increased in frequency when there was a mount present, consistent with previous research. We discuss these results in the context of how reliability of such a low-cost signal can be maintained, and propose new hypotheses for future research.