We use foraging theory to derive a priori predictions regarding how prey abundance, search efficiencies, and habitat structure should affect resource exploitation. We apply this conceptual framework to the rare and highly restricted Amani Sunbird (Hedydipna pallidigastra) and the widespread Collared Sunbird (H. collaris), that co-occur within a woodland habitat. We observed foraging sunbirds and recorded foraging rates (hop rate, flight rate and attack rate), food items per search movement, and food items per patch as influenced by bird species, tree species and foraging height. We evaluated arthropod abundance among tree species and at different heights within trees.
Results/Conclusions
Compared to the Collared Sunbird, the Amani Sunbird foraged higher in the canopy. Foraging rates of the Amani Sunbird was three times greater within the understory than within the mid- or upper- canopies. The Collared Sunbird had lower feeding rates across all three foliage strata. From foraging theory, the Amani Sunbird experience higher foraging costs at the understory than at the upper strata. The Collared Sunbird experience lower foraging costs across the three vertical strata. It appears that interference competition from the Collared Sunbird within the lower strata raises the foraging costs of the Amani Sunbird. Furthermore, the Collared Sunbird took fewer food items per patch, supporting a cream skimmer strategy of finding and exploiting rich patches by moving frequently from place to place. The Amani Sunbirds crumb picker strategy manifested by more food items per patch, reduced travel rates, and increased prey attacks within a patch. Arthropod densities were higher in Brachystegia spiciformis and Hymenaea verrucosum than in the other six tree species.
