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

Abstract

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.

Pedal Claw Curvature in Birds, Lizards and Mesozoic Dinosaurs – Complicated Categories and Compensating for Mass-Specific and Phylogenetic Control

“…The correlation is poorer than that reported in previous studies [4], [66], with behavioural categories showing opposite trends to the results found here. The claw mid-point height, however, correlates well with mass, similar to the result reported by Pike and Maitland [4].…”

“…Manning et al [32] concluded that “the feet and hands of dromaeosaurs functioned both for locomotion (walking, running, and climbing) and as prey capture/grappling devices”, though our data for digit III supports only a terrestrial interpretation. Deinonychus , however, does fall within the predatory category, suggesting that its foot may have been used in prey immobilisation behaviour analogous to modern Accipitridae [66], [69].…”

Quantifying shape and ecology in avian pedal claws: The relationship between the bony core and keratinous sheath

“…Additionally, there was a previously unrecognized confounding factor present in the data, high intraspecific variation. Some previous studies used up to six specimens per species when examining claw shape (Birn‐Jeffery et al, ) while some other studies have used a single individual per species (Csermely et al, ). When we examined several species that had high sample sizes in our dataset ( n > 18) very high intraspecific variation was observed, whether using traditional measurements or geometric morphometric analyses (Figure ).…”

“…Taxa were then split into three ecological groups (predominantly predatory, predominantly ground‐dwelling, and flying generalists) to assess how claw shape related to ecology. Birds of prey (e.g., Accipitriformes, Strigiformes, Falconiformes) were classified as predominantly predatory birds given the unique ways that they use their claws in prey capture (Brown & Amadon, ; Csermely et al, ; Csermely & Rossi, ; Del Hoyo, Hoyo, Elliott, & Sargatal, ; Johnsgard, ). Birds were considered predominantly ground‐dwelling birds if they spend the majority of their time, or all of their time, on the ground (e.g., ratites, bustards, some Galliformes; Del Hoyo et al, ).…”

“…Pairwise comparisons were performed on species means for each traditional metric using phytools in R (Revell, ). All PGLS models included size as a factor, as some previous studies have found a significant impact of size on claw arc metrics (Birn‐Jeffery et al, ; Csermely et al, ; Pike & Maitland, ). While body mass is commonly used as a size metric, museum specimens do not usually have body mass recorded and only 89% of the included taxa had known body masses in the literature (Dunning, ).…”