Meeting Abstract
With nearly 11,000 currently recognised species, birds (Class: Aves) are the most diverse clade of tetrapods. They possess a highly modified skull compared to the non-avian dinosaurs, with exceptionally thin bone, lightweight construction, and considerable cranial kinesis. The most striking modification to bird skulls, however, is the beak: an expanded, toothless maxillary region covered by a keratinous rhamphotheca. Beaks are highly disparate, reflecting not only the diversity of birds, but also the wide range of feeding ecologies that they operate in. Yet even with such disparity, morphological convergence is rife, and similar beak shapes evolve repeatedly across the avian tree. Well-demonstrated links between beak shape changes and dietary shifts are often assumed to drive high rates of phenotypic evolution, largely thanks to the extensive study of such relationships in classic adaptive radiations like Galapagos finches and Hawaiian honeycreepers. However, these patterns are rarely quantified with a wider lens. By utilising molecular phylogenies and morphometric analyses of up to 5,550 species at a time, it becomes apparent that these ‘classic’ radiations are exceptional events that are not characteristic of avian evolution more broadly. Rates of beak shape evolution are not strongly tied to extrinsic factors, and instead are associated with phenotypically unusual clades ‘unlocking’ new niches. Within these niches, beak shapes tend to remain relatively conserved, but with significant phylogenetic, allometric, and integrative signals superimposed. Dietary behaviour, conversely, predicts very little shape variation thanks to extensive many-to-one relationships between function and form. Bird beak evolution must therefore be considered more holistically if we are to truly understand how diversity accumulated in this exceptional clade.
