Mixed-species groups (MSGs), also termed mixed-species flocks (MSFs), form widespread social assemblages in birds that can enhance foraging success and reduce predation risk for participants. While ecological and behavioral drivers of MSFs have been studied, the role of morphological and visual traits in mediating which species join flocks has received relatively little quantitative attention. This study investigates whether plumage colour structures MSFs at a global scale by testing whether birds that flock together are more similar in colour than expected by chance.
The authors framed two complementary questions: (1) are observed MSFs composed of species that are more similar in plumage colour than would be expected from the available flocking species at a site, and (2) do MSF participants represent a restricted subset of the regional colour space?
The analysis used observational data spanning four continents. The source reports a global sampling approach but does not list specific sites or the total number of flocks and species in this summary. The study compares observed flocks at each site to null expectations built from the local pool of flocking species available at that site.
To quantify visual similarity among flock participants, the authors developed a new metric that captures colour similarity among species within each observed MSF. The manuscript summary does not provide formulaic details of the metric in this abstract-level text; those specifics were not reported in the source provided here. The metric was applied consistently across sites to allow standardized comparison of observed flocks to null assemblages.
Observed MSFs were evaluated against null assemblages constructed from all flocking species at each site. The nulls represent random assemblages drawn from the local pool, permitting a test of whether observed flocks are non-random with respect to plumage colour. The source indicates that this approach was used across all sampled sites and that comparisons focused on the degree of colour similarity within observed MSFs versus expected similarity under the null model.
Across all sampled sites, birds participating in MSFs were more similar in plumage colour than predicted by the null assemblages. This consistent pattern indicates that plumage colour is a non-random structuring trait shaping flock assembly at a global scale. The findings support the inference that visual similarity among species contributes to which species co-occur in mixed-species flocks.
The strength and prevalence of colour structuring varied across geographic regions, according to the source. However, the analysis did not find a relationship between flock size and the degree of colour structuring; flock size did not explain variation in colour similarity among participants in the summary provided. The exact geographic patterns and statistical details were not reported in the abstract-level text available here.
Within local communities, MSF participants occupied a restricted region of colour space compared with the regional pool of species. The subset of colours represented in flocks was dominated by yellow and brown plumage. This indicates that, beyond pairwise similarity, MSFs sample non-randomly from the suite of available colours in the community, favoring particular plumage hues.
The study concludes that visual traits—specifically plumage colour—play an important role in structuring interspecific social systems in birds. By demonstrating that birds of similar plumage colour are more likely to flock together globally, the findings highlight that colour influences both overall participation in MSFs and the species composition within flocks. The authors interpret these patterns as evidence that visual similarity can reduce individual predation risk and thus act as a driver of interspecific social grouping.
The authors declared no competing interests. The preprint was posted August 26, 2026, on bioRxiv and is available under a CC-BY-NC-ND 4.0 International license.
Note: The source summary does not provide specific methodological formulas for the colour-similarity metric, the full list of sampling sites, or detailed statistical results; those details may be available in the full manuscript or supplementary materials but were not reported in the provided abstract text.