Functional Morphology of Raptor Hindlimbs: Implications for Resource Partitioning

Abstract

Prey capture in owls and hawks is largely dependent on the biomechanics of the hindlimbs, and both limb size and grip forces potentially determine the size of prey that can be captured and the extent of possible resource partitioning among sympatric species. Morphological study of six species of sympatric raptors-the owls Otus asio, Strix varia, and Bubo virginianus; and the hawks commonly considered their diurnal ''ecological equivalents,'' Falco sparverius, Buteo lineatus, and Buteo jamaicensis-revealed that, in both groups, talon closure is effected by two discrete mechanisms that function together in a potentially additive or alternative fashion. Grip force measurements obtained from live owls and hawks using ''hydraulic'' perches showed that grip force increases exponentially with body size and that owls produce greater forces than hawks. That finding is consistent with the distinctive osteology and myology of their hindlimbs and with their hunting behavior. These data provide some understanding of the different demands of diurnal and nocturnal hunting as well as the mechanism of coexistence for those six species in eastern woodlands.

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

“…However, other species in these studies were comprised of Buteo hawks and owls (e.g. Marti, 1974;Csermely and Gaibani, 1998;Ward et al, 2002). These species might be expected to produce greater grip forces than the Accipiter hawks and falcons of our study because of their larger body sizes, primarily mammal-eating habits, and, in the case of owls, nocturnal behavior and zygodactylous toe configuration (Ward et al, 2002;Einoder and Richardson, 2007b).…”

“…Marti, 1974;Csermely and Gaibani, 1998;Ward et al, 2002). These species might be expected to produce greater grip forces than the Accipiter hawks and falcons of our study because of their larger body sizes, primarily mammal-eating habits, and, in the case of owls, nocturnal behavior and zygodactylous toe configuration (Ward et al, 2002;Einoder and Richardson, 2007b). Grip forces of accipiters scaled with positive allometry in our study and in Ward et al (Ward et al, 2002), suggesting that allometric scaling of grip forces (and underlying digit flexor PCSA) may be a general characteristic of accipitrid hawks.…”

“…Falcons, however, tend to employ their jaws by delivering powerful bites to the neck to kill their victims by breaking the cervical vertebrae and damaging the spinal cord (Cade, 1982;Hertel, 1995) (but see . Previous studies have demonstrated how morphological characteristics of the beaks (Bierregaard, 1978;Hull, 1991;Hertel, 1994a;Hertel, 1995;Johansson et al, 1998;Dzerzhinsky and Ladygin, 2004) and hind limbs (Bierregaard, 1978;Hertel, 1994b;Ward et al, 2002;Einoder and Richardson, 2007a) of raptorial birds are related to differences in diet and/or feeding behavior across groups. For instance, Hertel (Hertel, 1995) found suites of skull, maxillary and mandibular characteristics that served to discriminate among trophic specialists; whereas bird-eaters tend to have wide skulls and beaks with increased mechanical advantage, scavengers tend have narrow beaks with greater curvature.…”

“…For instance, Hertel (Hertel, 1995) found suites of skull, maxillary and mandibular characteristics that served to discriminate among trophic specialists; whereas bird-eaters tend to have wide skulls and beaks with increased mechanical advantage, scavengers tend have narrow beaks with greater curvature. Ward et al suggested that the greater digit flexor area and grip-force performance of owls relative to similarly sized raptors provides a potential prey-partitioning mechanism between syntopic diurnal and nocturnal birds of prey (Ward et al, 2002). Irrespective of prey type, however, bite-and grip-force capabilities of raptors are fundamentally important for subduing and killing their prey in order to feed (Hertel, 1995).…”

“…In raptors, grip force has clear implications for their abilities to successfully subdue and kill their prey prior to feeding, and consequently has been investigated for some species [e.g. North American owls (Marti, 1974); American kestrels (Yamamoto, 1994); common buzzards and Eurasian kestrels Csermely and Gaibani, 1998); North American Buteo hawks, American kestrels and owls (Ward et al, 2002)]. Various characteristics of raptor talons play important roles in grasping, such as claw size and shape (Csermely and Rossi, 2006;Einoder and Richardson, 2007a;Fowler et al, 2009), and the digital flexor tendonlocking mechanism of the feet (Ward et al, 2002;Einoder and Richardson, 2006).…”