The hindlimbs play a crucial role in bird locomotion, making the biomechanical properties of the musculoskeletal system in these limbs a focal point for researchers studying avian behaviour. However, a comprehensive analysis of the mechanical performance within the long bones of hindlimbs during locomotion remains lacking. In the present study, the strain and deformation of the femur of Cabot’s Tragopans (Tragopan caboti) were estimated. We employed inverse simulation to calculate the force and moment of femoral muscles during mid-stance terrestrial locomotion and conducted finite element analysis to calculate femoral strain. Results showed that during mid-stance, the femur experiences combined deformation primarily characterized by torsion, bending, and compression. It emphasises the importance of considering the influence of varying loads on bone adaptation when investigating bone form-function relationships. Muscles were found to play a significant role in offsetting joint loads on the femur, subsequently reducing the deformation and overall strain on the bone. This reduction enhances femoral safety during locomotion, allowing birds to meet mechanical demands while maintaining a lightweight bone structure. Notably, the M. iliotrochantericus caudalis significantly reduces torsional deformation of the proximal femur, protecting the vulnerable femoral neck from high fracture risk induced by rotation load. Given that the femur torsion during terrestrial locomotion in birds is associated with changes in hindlimb posture due to their adaptation to flight, the characteristics of M. iliotrochantericus caudalis may provide insight into the locomotor evolution of theropods and the origin of avian flight.

Raptors share a common predatory lifestyle, but are different in food preferences and hunting behavior. The grip force and talons' grasping capabilities are fundamentally crucial for subduing and killing their prey to feed, but the abilities and differences to generate force are less known. In this study, the entire pelvic muscles were dissected with the muscle mass and fibre length measured and physiological cross-sectional area counted in the Common Kestrel (Falco tinnunculus), Eurasian Sparrowhawk (Accipiter nisus), and Long-eared Owl (Asio otus). Statistical tests were performed to explore the possible differences in architectural parameters among species. These species were same in distributing the greatest proportion of muscle mass to the shank region and the digital flexor functional group, allocating more than 60% muscle mass in relation to total single leg muscle mass to the same seven individual muscles including flexor digitorum longus (FDL), flexor hallucis longus (FHL), and tibialis cranialis (TC) which are three major muscles responsible for talon closure. Interspecies differentiations were most present in the shank and tarsus instead of other regions of the leg, which might reflect their difference in hunting mode and foot use. Greater force-generation capacity of FHL and some anatomical features suggest that digits 1 and 2 work together as an efficiently vise-like set, playing more critical role than digits 3–4 in foraging of diurnal raptors but to a different degree. In accordance with zygodactyl foot morphology, each digit of the Long-eared Owl plays a subequal role when hunting, evidenced by anatomical and architectural features. Because of its unique insertion to the base of the pygostyle, the striking numerical difference in the development of M. caudofemoralis was possibly related to raptors' flight behavior and feeding ecology. Concluded from anatomical and architectural aspects, the similarities and differences of the hindlimb musculature were correlated to common predatory lifestyle and different foraging behaviors in three raptor species. These results illustrated the underlying myological basis for the functional capacities of the leg muscles and may provide additional information useful in further biomechanical investigation and computer simulation.