Tree climbing – a fundamental avian adaptation

Abstract

There have been a number of studies on the claws of Mesozoic birds, largely driven by interest in the habitat of Archaeopteryx. Many Mesozoic avians have large, well formed manual claws, largely absent in contemporary birds. Juvenile hoatzins are the only living birds with claws that are large enough to be generally functional, but not equivalent to those of Mesozoic birds. When birds developed an effective backstroke permitting easy ascent from flat surfaces, the need for manual claws disappeared, which would suggest that they were primarily used for climbing tree trunks and had little function in prey capture. This hypothesis has both phylogenetic and functional implications. The numerous claw studies to date are based primarily on measurements taken of the bony core, all that is usually preserved in fossils. Examination of contemporary birds shows that this is a poor estimator of the size and shape of the horny sheath that actually forms the functional claw. The discovery of vast numbers of exceptionally preserved fossil birds from the Late Jurassic and Early Cretaceous of China means that we now have an opportunity to compare actual horny claw data from the earliest birds with that of modern birds and test hypotheses on climbing, terrestrial activity, and predation.

Keywords:

• claw morphology
• avian evolution
• arboreal
• claw angle
• Dromaeosauridae

Acknowledgements

We graciously thank E. Gong (Northeastern University) for access to specimens and providing support, and R. Timm for access to modern specimens under his care. We dedicate this manuscript to the memory of Cyril Walker, who realized the unique characters of enantiornithurine birds, and thank Gareth Dyke for organizing this symposium in his honour.