Is the Soft Song of the Brownish‐Flanked Bush Warbler an Aggressive Signal?

Abstract

Soft songs have been detected in many songbirds, but in most species, research on soft songs has lagged behind studies of broadcast songs. In this study, we describe the acoustic features of a soft song in the brownish-flanked bush warbler Cettia fortipes. Compared with the broadcast song, the warbler's soft song was characterized by a lower minimum frequency and longer duration, and it had a higher proportion of rapid frequency modulation notes. Using playback experiments, some in combination with mounted specimens, we found different responses to soft and broadcast songs, and we found that soft song can predict aggressive escalation (attack). We conclude that the soft song is an aggressive signal in this species.

Reliability of the Brownish-flanked Bush Warbler’s soft song in male-male conflict

“…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.…”

“…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.…”

“…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.…”

“…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.…”