Mixed-species bird flocks enhance the benefits of group aggregation by minimizing variation in body mass while maximizing variation in diet

Many animal species participate in interspecific groups which can provide benefits such as better detection of predator presence, but may also lead to costs such as interspecific competition. In interspecific groups, species with particular functional traits should be aggregated in a way that maximizes the benefits and minimizes the costs of these interactions. Further, the balance between costs and benefits depends on the resource availability and the spatial scale in which species interact. We aim to determine how species traits are distributed within and among interspecific groups and how intrinsic (group size) and extrinsic (environmental conditions) factors relate to aggregation patterns. We surveyed 192 bird species distributed in 355 mixed-species bird flocks along an environmental gradient in southwest Brazil and analyzed data at both scales of individual flocks (n = 355) and sites (n = 29). We used mixed-effect and multiple regression models to test if the functional richness of coexisting species changes with increasing group size and in response to environmental covariates that may influence the selection of specific traits. Using a null model approach, we inferred the differences between observed and simulated associations (random expectations). We found that groups with more than 20 species have greater diet variation (i.e. more omnivores) and are more similar in body mass than expected randomly. Indeed, larger groups consume more resources, increasing competitive dynamics and decreasing facilitation among species. Thus such groups should take advantage of a more profitable trait variation among co-occurring species. Our results suggest that the distribution of species in close associations is determined mostly by ecological processes occurring at local scales (i.e. not dependent on regional spatial context). Additionally, larger groups maximize foraging trait variation (diet diversity) and minimize predator scape variation (similar body mass) potentially increasing the benefits of the aggregation.

Automated summary

Many animal species participate in interspecific groups for benefits, but it can also lead to costs. The distribution of species within these groups depends on resource availability and spatial scale. A survey in Brazil found that larger groups have greater diet variation and are more similar in body mass than expected randomly.