Foraging modes of Mesozoic birds and non-avian theropods.

The origin and early evolution of birds has been a major topic in evolutionary biology. In the 20th century, evolutionary scenarios posited either ground-based bird ancestors or tree-dwelling ancestors. This has since been recognised as a false dichotomy [1]. We suggest that part of the problem is the loose categorisation of many extant bird species as either ground or tree locomotors when considering hind-limb function [2-7]. In reality these are not mutually exclusive alternatives. Many extant birds exhibit different degrees of ground- and tree-based behaviours. We thus propose they can be better placed on a spectrum - rather than a dichotomy - according to the extent of ground and/or tree foraging they exhibit. To test this system we analysed the toe claws of 249 species of Holocene birds, revealing that claw curvature increases as tree foraging becomes more predominant. Improved claw morphometrics allow more direct comparisons between extant and extinct birds in order to infer the behaviours of the latter. In contrast to previous studies [2-6], we find that claw curvatures of Mesozoic birds and closely related non-avian theropod dinosaurs, differ significantly from Holocene arboreal birds and more closely resemble those of Holocene 'ground-foraging' birds.