Grasping hold of functional trade-offs using the diversity of foot forms in Australian birds

Abstract

The remarkable diversity of ecological niches that birds have evolved to inhabit have resulted in their status as model organisms to study how the same morphological features can adapt to different environments. Not least of these features is the avian foot, which has diversified into a wide range of forms suited to several different ecological functions. In this investigation, we examine how a potential trade-off between two such functions is expressed in the foot morphology of the Australian avifauna; namely, the impact that specialising for either walking or grasping has on the proportions of the phalanges. The lengths of the body, foot, third digit and its phalanges, and the hallux were recorded from 106 preserved skins belonging to 22 species. Our analysis of these data shows that this functional specialisation presents a similar morphological gradient in Australian birds as has been previously observed in American species, with a few unique exceptions. Generally, species that are reliant on the foot to grasp (e.g., perching and gripping prey) display greater distal phalanx and hallux lengths than species that are specialised for walking or wading. However, the terrestrial Megapodes of Australia demonstrate a more intermediate morphology, potentially as a result of the unique mound construction behaviour occurring in this clade. These findings have relevance not only for use in identifying the ecology of cryptic or extinct species from morphology, but also for determining future evolutionary changes in different avian groups.