FORAGING ECOLOGY AND HABITAT UTILIZATION IN THE GENUS SIALIA

Abstract

Factors influencing foraging behavior are examined for the three species of Sialia in a variety of habitats. Searching for ground-dwelling prey from a perch is the fundamental predatory tactic of each species, but the proportion of insects in the diet and the frequency of aerial foraging vary interspecifically with wing-loading. S. currucoides occurs in several vegetation communities and is the most ecologically tolerant species, partly because it may forage by hovering in areas having low perch densities. Bioenergetic advantages of hover-foraging vis-a-visperch-foraging include increased capture opportunities in areas away from perches and an enlarged perceptual field afforded by the relatively great heights at which hovering occurs. S. sialis hybridizes with S. currucoides but does not hover-forage; it occurs at low elevations and is frequently found in oak ( Quercus sp.) woodlands. S. mexicana , rather limited in ecological tolerance, is usually found in ponderosa pine ( Pinus ponderosa ) forests and captures more of its prey above-ground than the other species. Two different species may occur in intermediate habitats, in which case the lack of major differences in foraging behavior and the importance of prey located in a two-dimensional (ground) space apparently make interspecific territoriality adaptive. Prey abundance, perch distribution, and density of ground cover are important habitat variables readily identifiable by the forest manager; changes in these variables have different effects on the various species and may alter the relative proportions of two coexisting species. Research supported in part by Frank M. Chapman Memorial Fund of the American Museum of Natural History.