Foraging ecology of Egyptian vultures in the Negev Desert, Israel.

Abstract

Egyptian vultures were observed at 3 kinds of feeding sites (randomly-placed sites stocked with 2 chicken carcasses, a fixed site stocked daily with 4 chicken carcasses, and a fixed site stocked 2x monthly with livestock carcasses) in the Negev desert, Israel, during breeding seasons of 1989 and 1990. Observations at large and small carcasses suggested Egyptian vultures were facultative social foragers; they invariably foraged in groups at predictable food supplies, but large flocks rarely gathered at small carcasses. Individuals did not recruit conspecifics to carcasses. Adults located more randomly-placed, small carcasses than other age-classes; at all sites they fed more intensively than nonadults and dominated them in aggressive encounters. These behaviors reflected the need to obtain more food in less time in order to feed and care for young. Egyptian vultures feeding at small-carcass sites had little competition from other species. Breeding adults made food deliveries to nests after feeding themselves. Adults fed out of proportion to their numbers because food items were small enough to defend effectively. Vigilance was strongly and consistently related to flock size. At the large-carcass site, griffon vultures and mammalian scavengers consumed the most food; Egyptian vultures experienced reduced and unpredictable access to food relative to small-carcass sites. Breeding adults made food deliveries to nests after gaining access to food, without feeding themselves first. Vigilance was unrelated to flock size, probably because other species determined access to food and risk of physical harm. Adults were unable to feed preferentially because food items were either too large (carcasses) or too small and diffuse (scraps, insects) to defend. Overall, most interactions of group and individual characteristics on individual feeding behavior were modified by site characteristics--chiefly perceived physical risk (due to unfamiliar surroundings or other competitors), food dispersion, and food availability. Supplemental feeding, an important tool for supporting threatened vulture populations, can benefit particular sizes or age-classes of vultures. Large vultures are favored by few, large carcasses with limited skinning. Small vultures are favored by small carcasses. Small vultures and subordinate vultures of all sizes are favored by many, easily accessed, well-dispersed food items.