Participants in mixed-species bird flocks (MSFs) have been shown to associate with species that are similar in body size, diet, and evolutionary history, suggesting that facilitation structures these assemblages. In addition, several studies have suggested that species in MSFs resemble each other in their plumage, but this question has not been systematically investigated for any MSF system. During the non-breeding season of 2020 and 2021, we sampled 585 MSFs on 14 transects in two habitats of Tongbiguang Nature Reserve in western Yunnan Province, China. We performed social network analysis and the Multiple Regression Quadratic Assignment Procedure to evaluate the effect of four species traits (body size, overall plumage color, distinctive plumage patterns, and diet) and evolutionary history on species association strength at the whole-MSF and within-MSF levels. All 41 significant relationships showed that species with stronger associations were more similar in their various traits. Body size had the strongest effect on association strength, followed by phylogeny, plumage patterns, and plumage color; diet had the weakest effect. Our results are consistent with the hypotheses that the benefits of associating with phenotypically similar species outweigh the potential costs of interspecific competition, and that trait matching can occur in plumage characteristics, albeit more weakly than in other traits. Several explanations exist as to why similarities in plumage may occur in MSFs, including that they could reduce predators’ ability to target phenotypically ‘odd’ individuals. Whether trait matching in plumage occurs through assortative processes in ecological time or is influenced by co-evolution requires further study.

One of the most fundamental goals of modern biology is to achieve a deep understanding of the origin and maintenance of biodiversity. It has been observed that in some mixed-species animal societies, there appears to be a drive towards some degree of phenotypic trait matching, such as similar coloration or patterning. Here we build on these observations and hypothesize that selection in mixed-species animal societies, such as mixed-species bird flocks, may drive diversification, potentially leading to speciation. We review evidence for possible convergent evolution and even outright mimicry in flocks from southwestern China, where we have observed several cases in which species and subspecies differ from their closest relatives in traits that match particular flock types. However, understanding whether this is phenotypic matching driven by convergence, and whether this divergence has promoted biodiversity, requires testing multiple facets of this hypothesis. We propose a series of steps that can be used to tease apart alternative hypotheses to build our understanding of the potential role of convergence in diversification in participants of mixed-species societies. Even if our social convergence/divergence hypothesis is not supported, the testing at each step should help highlight alternative processes that may affect mixed-species flocks, trait evolution and possible convergence. This article is part of the theme issue ‘Mixed-species groups and aggregations: shaping ecological and behavioural patterns and processes’.

Elongated tails are elaborate plumage traits possessed by a variety of bird species. Sexual selection has long been the dominant hypothesis for explaining why certain species possess such long tails. During the past three decades, however, alternative hypotheses have been proposed and tested. Here, we review evidence, and the lack thereof, for four hypothesised functions of elongated tails in birds: sexual selection, aerodynamic enhancement, anti‐predation signalling and perch balancing. Sexual selection has received the most attention, though with inconsistent support. The other three hypothesised functions, in contrast, have gained less attention, with perch balancing remaining largely speculative. After reviewing and synthesizing information about these functions, we show that our current knowledge of avian elongated tails is not comprehensive. To advance our understanding of this topic, we suggest (1) devising a uniform definition of elongated tails that can be applied to a wide variety of bird species, (2) further investigation of less‐studied functions of such tails, (3) conducting meta‐analyses to discover the origins and trait correlates of elongated tails, (4) considering the possibility that elongated tails serve multiple functions simultaneously, and (5) testing unexplored functions of elongated tails.