Hindlimb myology in two piscivorous raptorial birds: a quantitative comparison of the osprey and the white-tailed sea eagle (Aves, Accipitriformes)

The hindlimbs of Accipitriformes are vital for capturing prey. Research on hindlimb musculature has primarily focused on species preferring avian and mammalian prey, whereas piscivorous species have received limited attention. This study aims to investigate the quantitative characteristics of hindlimb muscles in two piscivorous Accipitriformes, Pandion haliaetus and Haliaeetus albicilla, to discern potential muscular features associated with their specific food preference. The mass and proportion of all hindlimb muscles in both species were assessed based on their primary function (flexion or extension). A Kruskal-Wallis test was employed to analyze possible differences in muscle mass between species. The allometric relationships between the muscles and body mass were explored with the reduced major axis method. Additionally, a study on the architectural parameters of the primary gripping muscles in P. haliaetus was conducted, using published information from other raptorial birds for comparison. The isometric scaling relationship predominated in the majority of individual muscles helping maintain a proportional relationship relative to body mass. Both species exhibited a similar pattern in terms of quantitative muscle features, implying a preservation of muscle characteristics linked to their predatory capabilities. The largest proportion of hindlimb muscle mass was dedicated to digit flexion in accordance with the grasping abilities of birds of prey. The muscles tibialis cranialis, flexor digitorum longus and flexor hallucis longus exhibited the greatest mass, high physiological cross-sectional area values, and long fibre, categorizing them as high-power specialists. These muscles are crucial in the flexion movements that produce the gripping action that characterizes birds of prey. Although no statistically significant differences were detected, each species displayed slightly distinct muscular characteristics, particularly in the architectural properties of the flexor muscles controlling digits II, III, and IV. These variations seem to be associated with differences in their prey preferences.