Most signals are thought to be reliable based on the underlying logic that receivers only respond to signals that are sufficiently reliable (Maynard and Harper 2003; Searcy and Nowicki 2005). If a signal is always unreliable, the receivers would ignore that signal. Thus, the signaling systems would not exist at all. When the signaler’s and receiver’s interests are opposed (e.g., in male-male competition for resources), there are two main explanations for the reliability of signals. First, some signals are reliable because those signals are forced to be honest due to physiological or anatomical constraints on signal production (Maynard and Parker 1976). For example, the dominant frequencies of calls used in male-male conflict in some frogs belongs in this category; the frequency is determined by the size of the vocal apparatus, which is determined by body size, and the dominant frequency is thus constrained to reflect reliable information about body size (Martin 1971, 1972). A performance constraint on the trilled vocalizations in Emberizidae is an additional example: maximal values of frequency bandwidth and trill rates are limited by motor constraint (Podos 1996, 1997). The second explanation for signal reliability is based on costs. Costs that stem from signaling can prevent deceit if the signaler’s fitness is decreased due to cheating (Grafen 1990). Birds often use vocal signals in territorial disputes, and the vocal signal used here might be associated with escalated contests (Searcy and Beecher 2009). If a weak individual cannot bear an escalated contest, reliable vocal signals could guarantee the cost of a conceivable escalated contest (Vehrencamp 2000; Molles and Vehrencamp 2001).
Although signals are often honest, unreliable signaling can exist in signaling systems (Maynard and Harper 2003; Searcy and Nowicki 2005). Unreliable signaling can be defined by departures from the typical correlation between the signal and an attribute of the signaler (Hauser 1996). For example, If signal a is typically correlated with action A, unreliable signaling occurs when an individual signals a but does not perform A (i.e., over-signaling) or does not signal a but does perform A (i.e., under-signaling). Depending on the value of the contested resource and the expected cost of an escalated contest for the signaler, models predict that unreliable signaling can be a useful strategy for some individuals or in some context (Szamado 2000; Szalai and Szamado 2009). For example, although song type matching (i.e., singing to the rival with the same song that the rival has just sung) has long be seen as an aggressive signal in song sparrows during male-male conflict (Stoddard et al. 1992), this signal can reflect the signaler’s expectation of an escalated contest in the initial of territory conflict but fail to predict direct attack (Searcy et al. 2006; Akcay et al. 2013). Because the reliability of a signal is context dependent, testing the signal in different contexts might aid the understanding of both the function of the signal and the evolution of the signaling system.
Soft song is characterized by markedly lower amplitudes than normal song (here termed broadcast song) and has long been observed in many birds (Dabelsteen et al. 1998; Morton 2000). Recent studies have confirmed that soft song is an aggressive signal; soft song can predict aggressive escalation in both passerine (Searcy et al. 2006; Ballentine et al. 2008; Hof and Hazlett 2010; Akcay et al. 2011) and non-passerine birds (Rek and Osiejuk 2011). Additionally, playback experiments with mounted specimens have shown that soft song is a reliable aggressive signal in male-male conflict (Searcy and Beecher 2009; van Staaden et al. 2011; Searcy et al. 2014). In a previous study, we found that the relationship between soft song production and subsequent attack is significant in the Brownish-flanked Bush Warbler Cettia fortipes (Xia et al. 2013b). Here, we examined the correlation between soft song production and subsequent attack in different male-male conflict contexts in this species.
The brownish-flanked bush warbler is a small, furtive passerine found throughout Southeast Asia. During the breeding season, the males sing clear, high-pitched broadcast songs from dense undergrowth (del Hoyo et al. 2006; Kennerley and Pearson 2010). The males have a small repertoire of broadcast songs (typically two or three songs) and always sing the broadcast song types in alternating patterns (e.g., a-b-a-b; Xia et al. 2013a). Compared to broadcast song, soft song in this species is characterized by lower amplitude, a significantly lower minimum frequency, more notes, a longer duration, and a higher note rate (Xia et al. 2013b). Thus, soft song is easily distinguished from broadcast song in the field. Based on our observations, broadcast song is always used in spontaneous singing and after expelling an intruder, while soft song is used during encounters between rival males. In a previous playback study (Xia et al. 2013b), we used mounted specimens positioned within the territory of subject males and conducted playback trails of broadcast songs and found that six of 25 territorial owners attacked the mounted specimen, and all attacking subjects generated soft songs immediately prior to the attacks. However, the context of the playback experiment of Xia et al. (2013b) is uncommon in the field because in contrast to broadcast song, soft song was always produced by the intruder when in close contact with the territory owner. The first aim of this study was to investigate whether soft song and subsequent attack are correlated in the territorial owner when the intruder produces soft song. The context of this study is more similar to that of the field when the territory owner closely encounters the intruder. Thus, this context can provide additional information about the function of soft song for birds.
The second aim of this study was to investigate the cost of soft song in the Brownish-flanked Bush Warbler. For signals with no obvious physiological or anatomical constraints, cost should be the most probable explanation for the stability of the signaling system (Vehrencamp 2000; Molles and Vehrencamp 2001). A previous study showed that soft song can induce male brownish-flanked bush warbler to spend significantly more time near the loudspeaker (Xia et al. 2013b). We hypothesized that closeness is a cost to the signaler in the experiment detailed by Xia et al. (2013b) based on the underlying logic that increasing closeness increases the possibility of attack. However, this underlying logic has not been verified in this species. In the present study, we show direct evidence of cost to the soft song signaler; i.e., soft songs induced more attacks.