Song type matching is an honest early threat signal in a hierarchical animal communication system

Abstract

Aggressive encounters between animals often involve significant amounts of signalling before or in lieu of physical fights. When, as is often the case, these apparent threat signals are neither inherently costly nor inherently indicative of fighting ability, we should ask whether they are in fact honest signals, i.e. do they predict that escalation is imminent? While signalling theories have indicated that such 'conventional' threat signals can honestly predict escalation, attempts to gather supporting empirical evidence have mostly failed. For example, recent studies in songbirds of song type matching (replying to an opponent's song with the same song type he has just sung) have failed to confirm that it predicts an eventual attack by the signaller. In the present study of song sparrows (Melospiza melodia), we tested the hypothesis that song type matching is an early threat signal in a hierarchical signalling system. We used an improved model-playback design that simulated an escalating intrusion onto the subject's territory: the simulated opponent first sang in hiding from the boundary before moving to the centre of the territory, where he revealed himself and continued to sing. We found that type matching beginning in the boundary phase and continuing into the escalation phase, or beginning immediately after the escalation, reliably predicted both subsequent escalated signalling (soft songs and wing waves) and subsequent attack on the model, supporting the hypothesis that type matching is a reliable early threat signal.

Individual differences affect honest signalling in a songbird

“…'type matched'). Of these four signalling measures, the first two have been shown to reliably predict attack during simulated intrusions with a mount [22,27]. Type matching also has been shown to predict attack, but only in the context of a gradual escalation and not when the simulated intrusion starts from the territory centre [27].…”

“…Although these studies have found that some putative threat signals are indeed correlated with concurrent or subsequent aggressive behaviour, the relationship between signalling levels and actual aggression is far from perfect. For example, in our recent study on song sparrows (Melospiza melodia), we found that while the level of low amplitude ('soft') songs on average reliably predicted whether or not a song sparrow would attack a taxidermic mount later in the trial, a substantial number of birds that signalled at a high level did not attack, whereas others that signalled at a low level (or not at all) did attack [27], see also [22]. Soft songs and wing waves were able to correctly predict only 66.7% and 62.3%, respectively [27]-significantly different from chance, but far from perfect.…”

“…For example, in our recent study on song sparrows (Melospiza melodia), we found that while the level of low amplitude ('soft') songs on average reliably predicted whether or not a song sparrow would attack a taxidermic mount later in the trial, a substantial number of birds that signalled at a high level did not attack, whereas others that signalled at a low level (or not at all) did attack [27], see also [22]. Soft songs and wing waves were able to correctly predict only 66.7% and 62.3%, respectively [27]-significantly different from chance, but far from perfect. The fact that aggressive signals predict aggression only imperfectly suggests that other uncontrolled factors may also be affecting aggression and aggressive signalling.…”

“…Of these four signalling measures, the first two have been shown to reliably predict attack during simulated intrusions with a mount [22,27]. Type matching also has been shown to predict attack, but only in the context of a gradual escalation and not when the simulated intrusion starts from the territory centre [27]. We also recorded the number of broadcast ('loud') songs which are known not to correlate with aggression in song sparrows (or in most songbirds for that matter; review in [33]).…”

“…Briefly, in the 2011 experiment, subjects were confronted with a playback sequence that started at the boundary of their territory followed by the revealing of a taxidermic mount along with playback at the centre of the territory for 15 min (or until attack). Attacks were defined as landing on or coming within 10 cm of the mount (for details of this experiment see [27]). We used these additional trials to determine: (i) how stable aggression is over this longer time period (more than a year), and (ii) whether our measures of aggressive behaviour did in fact predict attack (i.e.…”