Researchers from the Doñana Biological Station (EBD-CSIC) have discovered that certain visual and acoustic traits can help predict which bird species play a central role in tropical mixed-species flocks, temporary associations in which birds of different species travel and forage together. Based on more than 3,400 mixed-species flocks recorded across nine Neotropical countries, the study shows that the most “social” species, those that appear most frequently in these groups and associate with the greatest number of other species, tend to produce higher-pitched vocalizations, display more conspicuous plumage, and have relatively smaller eyes.

Many bird species, particularly those inhabiting South American forests, form these mixed-species flocks. Such alliances may provide a range of benefits, including enhanced protection against predators and improved foraging efficiency.

Some act as true “leaders”: they guide flock movements, issue alarm calls, and help maintain group cohesion. Others participate more peripherally, functioning as satellite species around these core or nuclear species.

“We wanted to understand why certain species occupy central positions within these social networks and whether specific physical traits are associated with that role,” explains Vicente García-Navas, researcher at the Doñana Biological Station and lead author of the study.

Bird social networks

To address this question, the researchers applied network analysis tools similar to those used to study human social networks. They analyzed thousands of observations of tropical birds compiled from studies published in both local and international ornithological journals.

Each species was treated as a node connected to other species according to how frequently they associated within mixed-species flocks. The researchers then calculated various indicators of centrality and connectivity to identify which birds functioned as genuine social hubs. These metrics were subsequently related to a range of traits, including relative eye size, beak shape, maximum song frequency, and plumage coloration.

The most central birds are more visible and audible

The results revealed clear patterns. Species with a greater tendency to participate in mixed-species flocks produced higher-frequency songs and exhibited a larger proportion of carotenoid-based plumage colors (yellows, oranges, and reds). In addition, species with more white in their plumage occupied more central positions within the network of bird associations.

According to the authors, these traits may facilitate communication and recognition among individuals of different species. More visible plumage or easily distinguishable acoustic signals could help maintain flock cohesion and improve the transmission of information about threats or movements.

Another notable finding was that the most social species tended to have relatively smaller eyes. This supports the so-called “many eyes” hypothesis: in larger groups, individuals can rely less on their own vigilance because other group members also help detect predators. In other words, membership in mixed-species flocks may reduce evolutionary pressure on certain individual sensory systems.

“The study suggests that some visual and acoustic phenotypic traits may have evolved specifically to facilitate life in mixed-species groups,” says García-Navas.

The authors highlight that “nuclearity”, that is, a species’ structural importance within the group, could itself be considered an evolutionary trait, comparable to morphological or physiological characteristics. The study also demonstrates the enormous potential of combining behavioral ecology, evolutionary biology, and network analysis to better understand how species cooperate in nature.