No evidence for a role of trills in male response to territorial intrusion in a complex singer, the Thrush Nightingale

Abstract

Among the broad diversity of songbird vocalisations, song can serve a wide range of different functions depending on the species and context. In many species, aggressive motivation has often been linked with the use of fast repeated series of elements typically referred to as trills. However, only a few studies explored the role of this specific component in species with a large repertoire and high song complexity. Here, we investigate the potential role of trills in the territorial vocal response of males Thrush Nightingale (Luscinia luscinia), a species with complex songs characterised by the frequent use and diversity of their trills. We performed playback experiments simulating territorial intrusion to test if trills signal aggressive motivation in this species. If so, we expected tested males to respond by changing their trill rate or frequency of trill use in songs, and/or using different trill types than before the stimulation. Contrary to our expectation, males did not modify their trill rate or differed in trill type use before, during or after playback. There was a tendency for decrease in trill duration and the number of elements in the trills during the playback stimulation, reflecting to some extent the overall use of shorter songs by males during that stage of the experiment. Altogether, our study does not support the role of trills as motivation signals for territory defence in Thrush Nightingales, but advocates for more research on their potential roles in signalling other information.