Musculoskeletal underpinnings to differences in killing behavior between North American accipiters (Falconiformes: Accipitridae) and falcons (Falconidae)

Abstract

Accipiters (Accipiter spp.) and falcons (Falco spp.) both use their feet to seize prey, but falcons kill primarily with their beaks, whereas accipiters kill with their feet. This study examines the mechanistic basis to differences in their modes of dispatching prey, by focusing on the myology and biomechanics of the jaws, digits, and distal hindlimb. Bite, grip, and distal hindlimb flexion forces were estimated from measurements of physiological cross-sectional area (PCSA) and indices of mechanical advantage (MA) for the major jaw adductors, and digit and tarsometatarsal flexors. Estimated bite force, total jaw adductor PCSA, and jaw MA (averaged over adductors) tended to be relatively and absolutely greater in falcons, reflecting their emphasis on biting for dispatching their prey. Differences between genera in estimated grip force, total digit flexor PCSA, and digit MA (averaged over inter-phalangeal joints and digits) were not as clear-cut; each of these parameters scaled positively allometric in accipiters, which may reflect the scaling of both prey size, and the proportion of mammalian prey consumed by this lineage with increasing body size. Estimated tarsometatarsal force was greater in falcons than in accipiters, due to their greater MA, which may reflect selection for incurring greater forces during prey strikes. Conversely, the comparatively lower tarsometatarsal MA in accipiters reflects their capacity for greater foot speed potentially necessary for grasping elusive prey. Thus, this study elucidates how differences in jaw and hindlimb musculoskeletal morphology of accipiters and falcons are reflected in differences in their killing modes, and through differences in their force-generating capacities.

In vivo bite and grip forces, morphology and prey-killing behavior of North American accipiters (Accipitridae) and falcons (Falconidae)

“…to determine the extent to which differences in their jaw and digital forces reflect their prey-killing behavior and musculoskeletal morphology. On the basis of their predatory behavior, as well as previous anatomical work (Sustaita, 2008), we expected that falcons would produce greater bite forces than accipiters, which, conversely, we expected to produce greater grip forces than falcons. A secondary goal of this study was to directly compare in vivo force measurements with Sustaita's (Sustaita, 2008) biomechanical estimates, both in their magnitudes and scaling relationships with body mass.…”

“…Sustaita investigated the mechanistic bases for differences in killing mode, focusing on the physiological cross-sectional areas (PCSAs) and moment arms of the digit flexors and jaw-closing muscles of falcons and Accipiter hawks (Sustaita, 2008). These two groups exhibit a similar range of body sizes and tend to specialize on avian prey, although certain members of each group take greater proportions of insect (e.g.…”

“…Differences in estimated grip forces between groups, however, were less pronounced due to differences in the scaling of digit flexor PCSA and mechanical advantage between accipiters (positively allometric) and falcons (isometric). Thus, differences in killing modes between accipiters and falcons are primarily reflected in their jaw forces, rather than in their grip forces, which only differ at the smallest and largest body sizes (Sustaita, 2008). The implication is that their feet play similar roles in subduing prey but that their beaks play substantially different roles for dispatching them.…”

“…Finally, we examined relationships between in vivo bite and grip forces and key external morphological variables, such as lower jaw and digit lengths. Given their roles as biomechanical out-levers for generating bite and grip forces (Sustaita, 2008), we expected that forces would decrease with increases in their lengths.…”

The Vascular Arrangement and Main Branches of Arteria Axillaris in Wing of Sparrowhawk (Accipiter nisus)

“…The sparrowhawk has got short and round tips of the wings, a long tail, a hooked beak and curved talons. Hindlimbs and claws are relatively more developed than beak in accipiters (Baris, 2004;Sustaita, 2008). Birds of prey which come from Europe and Asia migrate to Africa using the Eurasian-East African fly-way.…”