Signalling while fighting: further comments on soft song

Abstract

We thank Osiejuk (2011) for his comments on our discussion of the evolution of soft song in agonistic contexts (Akcay et al. 2011). Osiejuk raises several criticisms of one specific hypothesis we put forward in our paper, the readiness hypothesis. Here we discuss further some of the hypotheses on soft song, reply to Osiejuk’s criticisms and expand upon the readiness hypothesis. In our original discussion (Akcay et al. 2011), we outlined five hypotheses that have been discussed in the literature (see Table 1 for brief descriptions), and proposed a sixth one, the readiness hypothesis, that might explain why soft aggressive calls are soft. Under the readiness hypothesis, low-amplitude calls are thought to be a by-product of gearing up for a potential physical fight. In the case of songbirds, the need to track an opponent visually prior to a physical fight may preclude singing loudly (which requires that the head be thrown back). The same reasoning may apply to cases such as corncrakes, Crex crex (Rek & Osiejuk 2011), which despite calling nocturnally and in a dense habitat would probably still have to adopt an attacking posture and possibly locate the target visually before launching (or avoiding) a successful attack. As Osiejuk and we both noted, there are two interesting questions about soft song or calls. The first one is simply, why are they soft? This first question applies not only to aggressive soft vocalizations but also to any vocal signals that are low in amplitude. The second question is why is soft song so reliable a predictor of attack? This second question of course only applies to aggressive soft songs (Dabelsteen et al. 1998). The hypotheses put forward in the literature differ inwhich of these questions they seek to answer and their scope. Table 1 shows whether these hypotheses are generally applicable to soft vocalizations other than aggressive ones.