Functional differentiation in Neotropical mixed-species bird flocks is shaped more by phylogenetic structure than by flock richness

Abstract

Many bird species engage in flocking behavior, which can offer benefits such as improved predator detection but may also lead to costs arising from interspecific competition. The magnitude of these costs and benefits likely varies across species and depends on both environmental context and flock composition. Whereas extrinsic (environmental) drivers have been widely studied, the influence of intrinsic factors on the functional attributes of interspecific groups remains poorly understood. We examined the interspecific distribution of morphological, ecological, phenotypic (ornamental), and acoustic traits in mixed-species bird flocks from the Neotropical region. By adopting a phylogenetic community framework, we tested whether the level of among-species trait dissimilarity within a flock depends on the number of species it comprises (flock richness) and its phylogenetic structure (level of relatedness among species). Observed trait dissimilarity generally approximated null expectations, and significant departures from randomness were rare across traits. Within-flock dissimilarity in body size, beak shape, and maximum song frequency increased with richness until reaching a certain threshold (∼15 to 20 species); however, these relationships weakened after accounting for phylogenetic relatedness. Trait dissimilarity in diet and morphological traits increased with phylogenetic overdispersion. After controlling for richness, these relationships were only slightly attenuated for body size and beak morphology, whereas the association between dietary dissimilarity and phylogenetic structure became even stronger. The largely random assembly observed for most nontrophic traits is consistent with a decoupling of functional axes, whereby traits directly mediating resource use (Eltonian traits; body size, beak shape, and diet) exhibit stronger signatures of nonrandom assembly than traits associated with signaling or behavior. Our findings suggest that evolutionary divergence among co-occurring species contributes importantly to trophic differentiation within these micro-communities. Overall, this work underscores the importance of considering multiple trait dimensions when disentangling the relative contributions of stochastic and deterministic processes in structuring heterogeneous animal groups.