ADAPTIVE ADVANTAGES OF MIXED‐SPECIES FEEDING FLOCKS AMONG SEED‐EATING FINCHES IN COSTA RICA

Summary The feeding ecology of three Costa Rican finches occurring in mixed flocks, Tiaris olivacea, Sporophila aurita corvina and Sporophila torquella, was investigated by measuring both behavioural and ecological variables. Observations on foraging height, rate of hopping and pecking rate, as well as the identity, proximity and number of nearest neighbours were recorded. In addition the duration of all feeding and perching episodes were timed. Comparisons were also made between the abundances of food items (grass seeds) consumed and those potentially available for consumption. The analysis of variance of the feeding behaviour revealed that the presence or absence of neighbouring birds, whether of the same or different species, influenced the duration of feeding bouts more significantly than did either differences in habitat or species-characteristic behaviour. In addition the dietary comparisons revealed overlap in both species and size of seed consumed. Such similarities suggest that these species are not partitioning fields in the classical sense. We propose that the increase in the duration of the feeding bout associated with the presence of mixed species aggregations leads to increased feeding efficiency and is the result of intra- and inter-specific social learning. Certainly flocking is often advantageous, since searching in a group facilitates finding clumped resources; mixed species flocking, by increasing exposure to a diversity of foraging places and patterns, can further augment feeding efficiency.