Foraging Patterns of Nesting Gila Woodpeckers

Movement patterns, activities, and nest deliveries of Gila Woodpeckers (Melanerpes uropygialis) in nature were monitored in order to determine the mechanisms involved in substrate selection, the effects of distance from the nest on foraging behavior, and the ecological differences between sexes of this dimorphic species. The woodpeckers found insects in clumps, and tended to return to the same area and same substrate type after a successful delivery, but tended to switch substrates if no capture was made. They delivered saguaro pollen and fruit and preferred cacti with many fruits. Substrate selection also depended on the ambient temperature, however; at higher temperatures, the birds spent more time resting on the shady sides of saguaro cacti, where they had low rates of prey capture. Thus, while the birds selected substrates partly on the basis of searching efficiency, they were constrained by the physiological necessity of evading the desert heat. Optimal foraging theory predicts that to maximize the delivery rate to nestlings, foragers using small prey should select higher—quality patches, hunt longer, and deliver larger loads when farther from the nest. From greater distances, the Gila Woodpeckers did select more profitable substrates and deliver larger insects, as predicted by the theory. Visit duration increased only slightly with distance, but at greater distances, foraging effort was greater: the birds spent more of their time searching and pecking for insects rather than scanning or resting, had higher capture rates, and delivered a greater proportion of insects rather than fruits. The sexes differed in their foraging behaviors, in the selection of substrates, and in the composition of deliveries to the young. Males spent more of their foraging time pecking for insect larvae, whereas females spent more time searching for insect adults. Moreover, the sexes performed different parental roles; males spent much of their time guarding the nest, while the females primarily forged. This division of labor resulted in sexual differences in the effects of distance on substrate selection and on the activities performed on the substrates.