INTRASPECIFIC VARIATION IN ELEPAIO FORAGING BEHAVIOR IN HAWAIIAN FORESTS OF DIFFERENT STRUCTURE

I studied intraspecific variation in foraging behavior of an endemic, insectivorous bird, the Elepaio (Chasiempis sandwichensis), in two Hawaiian forests that differed in degree of human modification. The undisturbed forest had a closed canopy, a dense understory, and a groundcover of native plants. The disturbed forest had much lower tree and shrub densities, and a ground cover of alien grasses. Search-and-attack rates, proportions of attack maneuvers, and proportional substrate use differed between habitats. Birds in disturbed habitat attacked prey two-thirds as often as birds in undisturbed habitat, hopped less frequently, and new farther and more often. They also did less perch-gleaning and chasing, did more flight-gleaning and hawking, used small branches and the ground less often, and used leaves and the air more often than birds in undisturbed habitat. Disturbed areas may be lower-quality foraging habitat because they require more difficult foraging methods. Age was associated with variation in search-and-attack rates and proportions of attack maneuvers, but sex was not. Subadult Elepaio attacked prey less often than adults, searched more slowly, and used simpler maneuvers more often, possibly to compensate for their lower proficiency. Log-linear analysis showed that attack maneuver was related to substrate and to tree species. Birds perch-gleaned more often on twigs and in ohia (Metrasideros polymorpha), hung more often on bark and in koa (Acacia koa), and night-gleaned more often on leaves. Elepaio showed much flexibility in foraging behavior and used more-diverse attack maneuvers and substrates than related continental species, which may allow Elepaio to exploit disturbed habitats successfully.