The Composition and Foraging Ecology of Mixed-Species Flocks in Pine Forests of Hispaniola

Abstract

We determined the flocking propensity of 48 species of birds occurring in native pine forest in the Cordillera Central of the Dominican Republic, and the species composition of 180 mixed-species flocks. Flocks were unusually ubiquitous, with 46 species occurring in at least one flock, 11 species regularly present, and all insectivorous species and all migrant species participating. Most birds encountered were permanent residents, but winter residents (Nearctic migrants) were an important component of the flocks and, as a group, had the highest flocking propensity. Flocks were cohesive and the resident insectivore, the Black-crowned Palm Tanager (Phaenicophilus palmarum) often served as the nuclear species. Censuses suggest species richness within flocks reflects the species present in the habitat, but agonistic interactions indicate that intraspecific aggression may limit the number of individuals of a species in these flocks. Species co-occurrence data indicate that species do not occur independently of one another in flocks. Positive associations were far more common than negative co-occurrences, suggesting mutual habitat dependencies or species interactions within flocks. A non-random association of nearest neighbors also indicated that species may be gaining feeding benefits from flocking by associating as close neighbors with an individual of another species, but we were not able to rule out the possibility that predation is an important selective agent. Intraspecific comparisons of foraging behavior between flocking and solitary birds provides some evidence that individuals modify foraging locations and foraging tactics upon joining mixed-species flocks, and that their foraging behavior tends to converge with the feeding behavior of the nuclear species. An increase in the feeding rate was recorded for one species. These data suggest that at least some species may accrue feeding advantages as flock participants.