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

Abstract Mixed-species bird flocks are dynamic associations that exhibit compositional turnover over relatively small timescales. Morphological diversity and foraging behaviour of species in flocks influences the relative benefits and costs of flock participation. In addition, species within flocks are highly acoustically active. 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. We found that participant species are generally more similar in body mass than expected by chance. Flocks in general were dominated by smaller-sized species. Conversely, we found that flock participants are not similar in acoustic traits. Much literature suggests that morphology and acoustic signal parameters are known to be correlated, but we present evidence to suggest that these two trait spaces are 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. Lay summary Participants in mixed-species bird flocks tend to group together with similar-sized individuals. This morphological similarity in a crowded flock may result in acoustic signal overlap, as the two sets of traits are correlated to each other. Here, we find evidence to suggest that morphological and acoustic traits are decoupled in these interspecific associations, putatively enabling species to derive benefits from flocking with similar-sized species, and simultaneously minimize acoustic masking interference.