Effects of Plant Species and Foliage Structure on the Foraging Behavior of Forest Birds

We quantified the behavior of four species of passerine birds foraging for arthropods among the foliage of different plant species and vertical strata of a northern hardwoods forest in New Hampshire (USA). Two species (Vireo olivaceus and Setophaga ruticilla) often changed their foraging patterns among strata but not among tree species, except in white ash. In the latter, both species flew more frequently between perches while searching for prey, which reflects the more open canopy and sparser distribution of ash foliage. A third species (Vireo philadelphicus) was stereotyped in its search and attack methods but had variable attack rates, indicating greater foraging success in some microhabitats than in others. The fourth species (Dendroica caerulescens), which inhabits the forest understory, showed few significant differences in its search or attack behavior, despite considerable variety in the sizes, shapes, and arrangements of leaves among available plant species. The foraging patterns of S. ruticilla were further influenced by differences among tree species in the abundance of a major prey type (leafhoppers, Cicadellidae: Homoptera) and by the active escape behavior of those prey. We conclude that these four bird species, all of which search leaves at a distance and fly up to hover or snatch arthropod prey from leaf surfaces, are relatively unaffected by details of foliage structure such as leaf size, shape, petiole length, and arrangement on twigs. The distribution of foliage within a plant, however, and, in some cases, the abundance of certain food resources do differentially influence the foraging tactics and capture success of these species. Because foliage distribution and arthropod availability both vary with plant species, the floristics of a site will strongly influence the "foraging opportunities" available to birds and therefore their success in exploiting particular habitats.