Social Systems in a Tropical Forest Avifauna

The validity of the conclusion that predation is the primary force shaping social systems in the Monteverde avifauna rests on the assumption that the behavioral and morphological characteristics conferring reduced vulnerability, not food dispersion patterns, are the determinants of solitary foraging.

Abstract

An analysis of correlations between social systems, food disperson, and behavioral and morphological attributes related to predation vulnerability in a tropical avifauna indicates the following results and conclusions: (1) Solitary foraging is typical of species whose characteristics of activity, watchfulness, size, and habitat intuitively indicate reduced conspicuousness and vulnerability to relevant predators. (2) Active arboreal birds frequently forage in flocks. Their active search for food in the more open strata of the forest indicates greater vulnerability to predation in comparison to the rest of the avifauna. (3) Flocking is more probably an adaptation reducing susceptibility to predation than a development primarily determined by characterstics of food dispersion. Relative support is based on the number of exceptions to predicted correlations. (4) Among flocking species, flock composition is based to a large extent upon resource dispersion. Species with clumped resources flock intraspecifically; those using dispersed resources tend to be intraspecifically solitary and territorial, but interspecifically gregarious. (5) Thus, interspecific flocking occurs among predation-prone species involved in intraspecific contest competition such that intraspecific group sizes are too small to function optimally in reducing vulnerability. Interspecific flocking can be considered not as a special case but as a complementary, alternate strategy to intraspecific flocking in obtaining protection from predation. The validity of the conclusion that predation is the primary force shaping social systems in the Monteverde avifauna rests on the assumption that the behavioral and morphological characteristics conferring reduced vulnerability, not food dispersion patterns, are the determinants of solitary foraging. Confidence in the assumption is diminished somewhat by the occurrence among some solitary species of resource acquisition strategies that are not compatible with group foraging. However, these cases suggest that adaptations reducing vulnerability to predation have evolved by necessity with the adoption of specific resource-acquisition strategies requiring solitary foraging.