Comparison of claw geometrical characteristics among birds of prey and non-raptorial birds

It has long been held that Falconiformes and Strigiformes possess a strong similarity in their claws due to their adaptive convergence for preying specialisation and that claws differ from those of other bird species, justifying the usual name of talons, instead of claws. It is subjectively felt that talons in birds of prey are very similar and somewhat different from claws of other birds, but the analysis of the geometrical reason for that has been neglected so far. The aim of this study is to answer two questions: (1) whether raptor and owl talons are actually similar, (2) which parameter can describe the similarity or difference. The structure of toes and claws of first (toe-1) and third toe (toe-3) was evaluated in four groups: Strigiformes (suborder Strigi), Accipitridae, Falconidae, for the analysis of difference between birds of prey, and Non-raptorial species, used as out-group. One adult male per species was considered, and, among Non-raptorial birds, only species belonging to typically or partially perching families. Only one specimen per species was chosen, from museum skin collections. The species and families considered were those listed in a reliable worldwide checklist. The multiple discriminant analysis segregated Strigiformes from both Accipitridae and Non-raptorials. The Falconidae, however, had an almost symmetrical overlap on other groups. Function-1 separated Strigiformes from Non-raptorials, mainly involving the section shape of claw-1, claw-3, and of phalax-1, as well as how much claws are hooked in relation to their radius. Function-2 segregated Strigiformes from Accipitridae, basically involving the curvature of claw-1 and also that of claw-3 in relation to its toe length. Our results show that although apparently similar, owls’ talons differ in several characters from Accipitrids’ and Non-raptorials ones, while Falcons show intermediate characteristics, proving not to have particular specialisation in their talons, in accordance to recent phylogenomic studies.