The benefits of joining mixed-species flocks for Greater Racket-tailed Drongos Dicrurus paradiseus

Abstract

Drongos are well known to participate in mixed-species bird flocks, but it is not clear whether they are mostly commensalists, catching insects that other species disturb, or kleptoparasites, stealing food directly from other species. We studied the foraging ecology of Greater Racket-tailed Drongos Dicrurus paradiseus inside and outside mixed-species flocks in seven areas of a lowland rainforest in Sri Lanka. We found that drongos in all seven areas fed at a higher rate when in flocks. Kleptoparasitism was practised at a low rate (4% of observations); more often, drongos captured insects disturbed by other species (41% of observations). To accrue these benefits, drongos adjusted the height at which they perched to match the foraging height of Ashy-headed Laughingthrush Garrulax cinereifrons, a species that forages lower than drongos normally do, and those of Orange-billed Babbler Turdoides rufescens, a species that forages higher than drongos normally do. We conclude that drongos are better classified as commensalists than as parasites, as they exact only a small cost on other species in flocks; they may even be mutualists, because they make sensitive and reliable alarm calls to which other species react.