Frequency of Mixed Species Flocking in Tropical Forest Birds and Correlates of Predation Risk: An Intertropical Comparison

Abstract

The composition and frequency of occurrence of mixed flocks among foraging insectivorous birds were compared between several tropical forests from South America to the Indian Ocean and Southeast Asia, and related to forest structure, foraging behaviour and abundance of predators (diurnal raptors), to investigate relationships between Rocking behaviour and antipredator defence. Foliage gleaners, the most conspicuous and least vigilant foragers, were usually the most abundant flock members and the birds with the highest Rocking propensity. In contrast, species foraging low above ground or in dense vegetation were the least likely to join Rocks. Flocking propensity of all insectivores pooled, or of gleaners alone, generally declined from primary lowland forest to open (semi-deciduous), disturbed (logging gaps), managed or montane forest (broken canopy), i.e. when predator detectability was likely to increase. There was also a significant positive correlation between bird-hunting raptor diversity or abundance and Hocking rates of foliage gleaners or all insectivores. The results supported the hypothesis that foraging birds most vulnerable to predators were most likely to form mixed species flocks anti that Hocking tendency decreased when vegetation structure became more protective and/or when predator pressure became lower.