Skeletal and functional anatomy of Hemignathus wilsoni (‘Akiapōlāʻau), a Hawaiian honeycreeper with unique foraging and feeding behaviors

Abstract As a result of its climate, isolation, and topographical variation, the Hawaiian archipelago has produced one of the greatest examples of avian adaptive radiation in the Hawaiian honeycreepers. While the foraging behaviors and ecologies of some Hawaiian honeycreepers closely reflect those of continental species, others have evolved novel and unique morphologies unrepresented among other birds. Here we describe the skeletal anatomy and functional implications of Hemignathus wilsoni (‘Akiapōlāʻau)—often referred to as the Hawaiian woodpecker. The upper and lower bills of H. wilsoni are markedly different in shape, with the stout, short lower bill used to gouge into woody substrate, and the thin, elongate, and down-curved upper bill used to probe and extract prey. Our osteological examination reveals a substantial difference in the length and shape of the upper bony rostrum and the overlying rhamphotheca. Previously unrecognized, morphological differences in these two features signal a material properties trade-off during probing with specific areas of flexion. Morphology of certain muscle insertions in the lower leg signal strengthened dorsiflexion of the ankle relative to other cardueline finches, which aids in full-body assisted movement during woody substrate gouging. Additionally, the femur is proportionally short and robust compared to closely-related continental cardueline finches—a condition similar instead to mainland woodpeckers which aids in lowering the center of mass against perches when foraging. Overall, the skeletal anatomy of H. wilsoni reflects the effects of insular evolution and highly specialized foraging behaviors, while also retaining signals of its mainland cardueline finch ancestry.