Importance of tropical mixed-species flocks for migratory birds in shade-grown coffee: implications of foraging together

Group living is a widespread social strategy that allows animals to optimize foraging with dynamic trade-offs between reduced predation risk, enhanced foraging, and increased competition. Birds across terrestrial habitats form mixed-species flocks, where dozens of resident and migratory species forage together throughout the day. The effect of flock participation and group size on the foraging behavior of migratory and resident species is poorly known, particularly in the tropics. Using foraging observations, we examined the effects of flock participation on the foraging ecology of four species of Neotropical migrants and three resident species in shade-grown coffee plantations in the Colombian Andes in 2011. We showed that resident and migrant birds in mixed-species flocks largely increased the number of foraging attempts, expanded their foraging repertoire, and exhibited greater overlap in use of foraging maneuvers compared with solitary individuals. We found an effect of group size on foraging rate, with feeding attempts increasing monotonically or showing a hump-shaped pattern, and also found that movement rates increased with increasing flock diversity. These results support the idea that flock participation enhances foraging success and reduces perceived predation risk, but also show niche convergence among flocking species, along with higher movement rates and more agonistic interactions as group size increases. Finally, our results provide insight into the importance of mixed-species flocks for birds of conservation concern, including Cerulean Warblers (Setophaga cerulea), a Neotropical migrant shown to be highly restricted to foraging in flocks in their wintering areas.