Seasonality in the Function of Avian Mixed-Species Flocks in Madagascar

Abstract

I examined whether or not the two functions of mixed-species flocks, anti-predator defense and foraging efficiency, differ in seven regularly-participating species between the breeding and non-breeding periods in Madagascan forests. The rate at which I encountered mixed-species flocks was greater in the non-breeding period because six regular species increased their flocking propensity. However, another regular species, the Rufous Vanga, which did not change its flocking propensity was encountered less frequently in mixed flocks in the non-breeding period. During the breeding period, when food was abundant and predators such as raptors were active, anti-predator defense should have been a major concern for flocking birds. Then, mixed-species flocks formed as other species' followed breeding groups of Rufous Vanga as they exhibit a high degree of vigilance and warning against predators. Conversely, during the non-breeding period, when food resources were fewer and conspecific flock sizes were larger, increased foraging efficiency was likely to be the primary concern of flocking birds. Then, those species that utilized similar foraging substrates aggregated to form flocks and did not choose the Rufous Vanga as a foraging partner. Accordingly, the function of avian mixed-species flocks in Madagascar may change seasonally from one of anti-predator defense in the breeding period to one of increased foraging efficiency in the non-breeding period.