Do claw geometrical characters differ in Owls, Raptors, and Non-Raptorial species?

A long-held concept is 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. We all subjectively feel that talons in birds of prey are all very similar and even somewhat different from claws of other birds, but the analysis of geometrical reason for that has been neglected so far. The aim of this study was to answer two questions: (1) whether talons of raptors and owls are actually similar and (2) which parameter can describe the similarity or difference. The structure of digits and claws of the first- (digit-1) and third digit (digit-3) was evaluated in four groups of birds: Strigiformes, Accipitridae, Falconidae, for the analysis of difference among the 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. Species and families considered were those listed by Sibley and Monroe (1990) worldwide. The Discriminant Function Analysis segregated Strigiformes from Non-raptorials, while Accipitridae had a moderate overlapping area on both. The Falconidae, however, have an almost symmetrical overlap on Accipitridae and Non-raptorials. Function-1 separated Strigiformes from Non-Raptorials and involved constantly digit-3. Function-2 segregated Strigiformes from Accipitridae and from Falconidae and basically involved digit-1. Among the most important characters are the curvature of both claw-1 and 3, as absolute value and in relation to their own length and to their phalanx, and the section shape (thin or rounder) of claw-1 and digit-3’s last phalanx. Our results show that although apparently similar, Owls’ talons differ in several characters from Accipitrids’ ones, while Falcons show intermediate characteristics between Accipitrids and Non-raptorials, showing to be surprisingly less specialised in their talons than Accipitrids.