Singing softly at the edge: vocal amplitude and performance limits in star finch courtship

Abstract

Studies of birdsong have long focused on long-range signals used for attracting mates and defending territory, but recent research has highlighted the importance of soft songs in social bonding and pair maintenance. Even for species that sing softly, courtship is a context in which enhanced vocal output can be beneficial. Indeed, many bird species modulate their song amplitude according to the social context. However, it remains unclear how this amplitude plasticity interacts with the biomechanical demands of producing structurally complex vocalizations. Here, we investigated how song performance changes with the social context in the star finch, Bathilda ruficauda, an estrildid species that produces soft songs with highly complex syllables. Comparing undirected songs and female-directed songs, we examined whether males increase amplitude during courtship and, if so, whether this modulation induces changes in other spectrotemporal parameters due to parameter couplings and potential biomechanical trade-offs. We found that males produced female-directed songs with higher amplitude than undirected songs, but the difference was strikingly small. Notably, this amplitude increase was accompanied by a reduction in Wiener entropy, while other spectral parameters remained unchanged. These results suggest that star finches operate near a biomechanical ceiling where further increases in vocal amplitude would compromise syllable complexity. We propose that, in star finch courtship, structural song complexity may be prioritized over vocal amplitude. Given that star finch courtship occurs only at close range and also involves visual signals, the selection pressure for high amplitude may be relaxed. Overall, our findings highlight the functional diversity of birdsong and the need to broaden our understanding of avian vocal performance. Ultimately, we advocate overcoming the research bias regarding territorial birds with loud broadcast songs in order to gain a more comprehensive insight into the behavioural ecology of birdsong.