Benefits of foraging in mixed-species flocks depend on species role and foraging strategy

Abstract

Mixed-species flocks of birds involve species with different roles and foraging strategies and are ubiquitous in forest environments. Species roles can broadly be categorized into leaders that attract others to form the flock, as well as lead the flock's movement; and follower species. The diversity of foraging strategies occurring in a mixed flock reflects the diversity of participant species. One of the main benefits of participating in mixed-species flocks is increased foraging efficiency. However, feeding benefits remain under debate, probably because not all participants receive this benefit in the same way. For instance, leaders and followers may benefit differentially, and particular foraging strategies might additionally influence feeding benefits. To identify leader species and evaluate foraging benefits in relation to species roles and foraging strategies, we conducted 375 h of surveys in subtropical Andean-foothill forests across 3 years. We recorded species leading the movement of the flocks and the foraging efficiency of the constituent birds. Because leader behaviour is related to the ability to attract other participants, we developed a standardized network analysis that helped us to identify them. We tested and compared whether behavioural leaders presented a higher topological position of dominance within the network. We found 35 species behaving like leaders; most could change from leaders to followers depending on the flock composition. The most frequent leader species had a higher topological dominance in the network. The foraging efficiency of leader and follower species inside mixed flocks increased compared with birds outside them. To some extent, the foraging efficiency of birds inside mixed flocks depended on their primary foraging strategy. Foliage Invertivore and Frugivore–Invertivore species tended to increase their foraging efficiency while Bark Invertivore, Aerial Invertivore and Granivore–Invertivore species did not show changes in their foraging efficiency in flocks. Although the general trend among mixed flock participants was to increase their foraging efficiency, exceptions suggest that other benefits come into play, such as predator avoidance or feeding on more profitable prey items. Our study indicates that benefits gained from foraging in mixed flocks depend on a complex combination of species roles, species-specific traits, and facilitation mechanisms linked to the primary foraging strategy of bird species.