Food availability for forest birds: effects of prey distribution and abundance on bird foraging

Abstract

To evaluate availability of food for passerine birds in a northern hardwood forest, we measured the abundance and distribution of Lepidoptera larvae, a major food type, on the foliage of the dominant tree species and the foraging behaviour used by birds to capture prey from foliage substrates. Larval abundances averaged 2.1 per 400-leaf sample, and differed significantly among tree species and strata and within and between years. Although American beech had significantly higher larval densities, yellow birch hosted a greater proportion of free-living larvae, which were more readily available to foraging birds. Most larvae (78–91%) were found on leaf blades, with 62–75% on leaf undersurfaces. Bird species differed significantly in the frequencies at which they attacked prey on different foliage substrates (upper or lower leaf surfaces, petioles, twigs) and tree species. Several changed their foraging patterns from one tree species to another. Six of the 10 bird species directed attacks to substrates in proportion to the abundance of larvae there; the other 4 deviated from this pattern because of preferences for other kinds of prey or because of different abilities to perceive and capture prey from trees with differing foliage structures. Our results suggest that availability of food for forest birds is a function of (i) the types and abundances of prey present, which vary among tree species, (ii) the foliage structure and characteristics of the trees, which influence prey detectability and accessibility, and (iii) the morphological and behavioural abilities of each bird species to perceive and capture those prey. Food availability therefore cannot be assessed by simply measuring prey abundance alone and must essentially be determined separately for each bird species. Since each tree species provides a differing set of foraging opportunities for birds, the mix of tree species at a site, coupled with the arthropod resources they support, will influence which bird species can successfully exploit that habitat and hence will affect bird community structure.