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

Raptors exhibit a diversity of strategies to procure their prey but ultimately kill using their beaks and/or talons. Thus, bite and grip forces are ecologically important variables that have direct survival implications. Whereas hawks rely primarily on their feet for killing prey, falcons tend to employ their beaks. Consequently, falcons are expected to achieve relatively greater bite forces, and hawks are expected to generate relatively greater grip forces. Force estimates predicted from musculoskeletal morphology in a previous study indicated that falcons (Falco spp.) possess greater jaw force capabilities than accipiters (Accipiter spp.) but there were no clear differences in predicted grip-force capacity outside of differences in scaling. The objective of this study was to complement those results with measurements of in vivo forces by inducing captive and wild accipiters and falcons to bite and grasp force transducers. Bite force increased isometrically in both groups whereas grip force tended toward positive allometry. After adjusting for body mass, falcons produced greater bite forces, and accipiters produced greater grip forces. Thus, previous anatomical estimates of forces predicted the expected direction and magnitude of differences in bite forces but the overall greater in vivo grip forces of accipiters deviated from the pattern obtained from biomechanical estimates. Although the scaling relationships were similar between data sets, forces generated by live birds were consistently lower than those predicted from biomechanics. Estimated and in vivo jaw and digital forces were nevertheless correlated, and therefore provide an important link between morphology and killing behavior in these raptors.