It Takes All Kinds in Acoustic Communication: A New Perspective on the Song Overlapping Phenomenon

The social role of song in wild zebra finches

Male songbirds sing to establish territories and to attract mates.^1,2 However, increasing reports of singing in non-reproductive contexts³ and by females^4,5 show that song use is more diverse than previously considered. Therefore, alternative functions of song, such as social cohesion³ and synchronization of breeding, by and large, were overlooked even in such well-studied species such as the zebra finch (Taeniopygia guttata). In these social songbirds, only the males sing, and pairs breed synchronously in loose colonies,^6,7 following aseasonal rain events in their arid habitat.^8,9 As males are not territorial, and pairs form long-term monogamous bonds early in life, conventional theory predicts that zebra finches should not sing much at all; however, they do and their song is the focus of hundreds of lab-based studies.^{10,11,12,13,14,15,16,17,18,19,20,21,22} We hypothesize that zebra finch song functions to maintain social cohesion and to synchronize breeding. Here, we test this idea using data from 5 years of field studies, including observational transects, focal and year-round audio recordings, and a large-scale playback experiment. We show that zebra finches frequently sing while in groups, that breeding status influences song output at the nest and at aggregations, that they sing year round, and that they predominantly sing when with their partner, suggesting that the song remains important after pair formation. Our playback reveals that song actively features in social aggregations as it attracts conspecifics. Together, these results demonstrate that birdsong has important functions beyond territoriality and mate choice, illustrating its importance in coordination and cohesion of social units within larger societies.

Song overlapping, noise, and territorial aggression in great tits

Communication often happens in noisy environments where interference from the ambient noise and other signalers may reduce the effectiveness of signals which may lead to more conflict between interacting individuals. Signalers may also evolve behaviors to interfere with signals of opponents, for example, by temporally overlapping them with their own, such as the song overlapping behavior that is seen in some songbirds during aggressive interactions. Song overlapping has been proposed to be a signal of aggressive intent, but few studies directly examined the association between song overlapping and aggressive behaviors of the sender. In the present paper, we examined whether song overlapping and ambient noise are associated positively with aggressive behaviors. We carried out simulated territorial intrusions in a population of great tits (Parus major) living in an urban–rural gradient to assess signaling and aggressive behaviors. Song overlapping was associated negatively with aggressive behaviors males displayed against a simulated intruder. This result is inconsistent with the hypothesis that song overlapping is an aggressive signal in this species. Ambient noise levels were associated positively with aggressive behaviors but did not correlate with song rate, song duration, or song overlapping. Great tits in noisy urban habitats may display higher levels of aggressive behaviors due to either interference of noise in aggressive communication or another indirect effect of noise.

A superb solo, or a deviant duet? Overlapping songs in superb fairy-wrens

Avian duets are formed when 2 birds coordinate their songs. Most research on the evolution and function of duetting has focused on species with highly coordinated duets, and less is known about the context and function of overlapping songs that are more loosely coordinated, in part due to the challenge of determining whether such vocalizations coincide by chance or through coordination between the partners. Here, we use field recordings and playback experiments to test whether breeding pairs of superb fairy-wrens, Malurus cyaneus, coordinate their territorial songs to form duets. We test 3 key characteristics of duetting; whether partners’ songs 1) overlap more than expected by chance; 2) have a stereotyped structure that occurs repeatedly and predictably in time, and 3) show evidence of a constant time lag between the contributions of the 2 participants, indicating that individuals are coordinating their songs. This is the first study to quantify the temporal precision of song between partners to investigate coordination in the Malurus genus, an important model taxon for song, sexual selection, and speciation. We found variation in the extent to which partners’ songs overlapped, with some individuals overlapping their partners’ songs more than expected by chance, no difference in structure of solo and overlapping songs, and no evidence of a consistent response interval. Thus song overlap in superb fairy-wrens meets only some criteria for duetting. We suggest that overlapping songs in this species may be due to individuals responding independently of the same stimulus and/or “call and answer” between pair members.