High association strengths are linked to phenotypic similarity, including plumage color and patterns, of participants in mixed-species bird flocks of southwestern China

Abstract

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.

Can convergence in mixed-species flocks lead to evolutionary divergence? Evidence for and methods to test this hypothesis

“…Beyond diet and body size, however, an observation repeated over the past 50 years and across the world is that species in MSFs often resemble each other in plumage [13,14]. Resemblances in plumage have been repeatedly suggested among members of passerine-dominated MSFs [23][24][25][26][27][28][29], as well as in MSFs of shorebirds [30] and parrot aggregations [31].…”

“…Flocking is widespread in China, though we are only beginning to rigorously examine the issue of phenotypic similarities within these MSFs (e.g. Zhou et al [29]). Using the online database 'Birds of the World' [39], we found 54% of the 652 passerine species of China have been described as associating in MSFs to at least some degree.…”

“…Finally, one resemblance among Pitohui species (Family: Pachycephalidae) in MSFs of Papua New Guinea appears to be because a model species is toxic [ 38 ]. Importantly, resemblances in MSFs need not be complete, but can come at various levels of shared traits, such as wingbars, crests or belly colour [ 29 ].…”

“…Zhou et al . [ 29 ]). Using the online database ‘Birds of the World’ [ 39 ], we found 54% of the 652 passerine species of China have been described as associating in MSFs to at least some degree.…”

“…MSF members are no more similar to each other than to other members of the community for a specific trait), then MSF membership is probably independent of phenotype. While assessing this is challenging, identification of species that are participating in MSFs, as well as species occupying the same habitat that are not participating in MSFs, will provide data to address this (see [ 29 ] for a quantitative examination of phenotypic matching, including plumage traits, at Tongbiguang Nature Reserve, Yunnan in southwestern China). Even more detailed observations can determine whether within microhabitats (e.g.…”