Contrasting morphological and acoustic trait spaces suggest distinct participant benefits in mixed-species bird flocks

Abstract

Mixed-species bird flocks are dynamic associations that exhibit compositional turnover over relatively small timescales. Morphological similarity and foraging behaviour of species in flocks influence the relative benefits and costs of flock participation. In addition, acoustic signals serve many critical life-history functions in birds, including within flocks. However, the role of acoustic signals in flock assembly remains poorly understood. Here, we examined the relationship between acoustic and morphological trait spaces of bird flocks in Peninsular India, using a phylogenetically informed framework. We found that participant species were generally more similar in body mass than expected by chance, and flocks in general were dominated by smaller-sized species. Conversely, we found that flock participants did not exhibit this pattern of similarity for acoustic frequency traits. Thus, in spite of the correlation of vocal frequency with traits such as body size and bill size, body mass and spectral signal space were decoupled at the community scale. This may enable species to derive distinct sets of benefits from both sets of traits and provides valuable insight into the dynamic processes driving flock assembly.