Quiet threats: soft song as an aggressive signal in birds

Abstract

Theory suggests that aggressive signals must be costly if they are to be reliable. Recent research in birds has shown, however, that in many species the best predictors of impending attack are low amplitude vocal signals, soft songs or soft calls, that seem cheap to produce and easy to cheat. This observation leads to two related but separate questions: 1) Why use low amplitude signals to communicate aggressiveness? and 2) What maintains the reliability of soft signals of aggression? We review potential answers to both questions and present evidence relevant to each. While some hypotheses are logically sound, others have logical flaws, and most of the hypotheses have yet to be critically tested. One exception is the hypothesis that the reliability of soft signals of aggressiveness is maintained by receiver retaliation, which has been supported by experimental evidence in multiple species. We emphasize the need for further research, particularly to answer the question of why soft song is soft, and outline future research directions.

Large vocal repertoires do not constrain the dear enemy effect: a playback experiment and comparative study of songbirds

“…Soft songs were of unusually low amplitude and fairly easy to identify in the field after spending time observing red-eyed vireos throughout the 2014e2015 field seasons. Soft songs have received recent attention because they may be associated with aggressive intent in several songbird species (Akçay, Anderson, Nowicki, Beecher, & Searcy, 2015). There are no previous reports of soft song in red-eyed vireos; however, our preliminary observations suggested that they occur in this species, just as they appear to be common among many other songbirds (Dabelsteen, McGregor, Lampe, Langmore, & Holland, 1998;Reichard & Welklin, 2015).…”

“…Soft songs have received substantial attention recently, but little is known about why birds use them to indicate aggressive intent and how these signals remain reliable (Akçay et al, 2015). Despite this, a wide array of animals, including birds (Akçay et al, 2015) and some mammals (Gustison & Townsend, 2015), are known to use soft vocalizations in aggressive contexts, and red-eyed vireos now join these animals. Two measures of red-eyed vireos' playback responses showed no differences in response to neighbours versus strangers.…”

Soft songs in male ortolan buntings are used in an aggressive context but are not an aggressive signal

“…The low amplitude of these signals appears to be easy to explain, and the reasons for limiting the active space of the signal are rather obvious. On the other hand, some bird species produce soft songs or calls during aggressive interactions as a means of territorial defence (e.g., Akçay, Anderson, Nowicki, & Beecher, ; Ręk & Osiejuk, ; Searcy & Nowicki, ; Xia, Liu, Alström, Wu, & Zhang, ). The link between signal function and low amplitude in this context is not clear.…”

Female House Wrens may Use a Low‐Amplitude Call as an Aggressive Signal

“…However, there is currently an open debate over why low‐amplitude rather than high‐amplitude signals are the most aggressive signal in many songbird species, especially as the latter seems more intimidating (Akçay et al. ; Reichard & Anderson ). Early work suggested males’ benefit from using low‐amplitude signals that limit eavesdropping by predators and other conspecifics (Dabelsteen et al.…”