Climate influences bill shape diversification in cavity-excavating birds

The remarkable structural and functional diversity of the avian bill serves as an ideal system to understand how ecological and environmental factors influence morphological diversification under physically demanding tasks. For example, in cavity-excavating birds where the bill experiences tremendous physical stresses, climate and habitat affect the mechanical properties of available nesting substrates and therefore can potentially influence the shape and performance of bills. Few studies have focused on the entire avian bill, a complex structure consisting of the rhamphotheca and bony core of the upper and lower mandibles, which may evolve as independent modules or as a single integrated unit. Here, we examined patterns and drivers of bill shape diversification in barbets using micro-computed tomography of museum study skins. We employed landmark-based geometric morphometrics and phylogenetic comparative methods to study the influence of climatic conditions, integration and phylogenetic constraints on bill shape diversification. Asian and African barbets have explored distinct maxillary shape spaces without corresponding change in mandibular shape after correcting for phylogenetic non-independence. Additionally, maxillary shape for both rhamphotheca and bony components was significantly correlated with climate variables and exhibited signatures of allometric scaling. All four bill components, however, covaried with each other and have evolved in a gradual and integrated fashion, exhibiting late-stage shape disparity. Through these findings, our study offers crucial insights into how ecological behaviors impact morphological evolution of complex, multifunctional features.