Form and function in the bills of cavity-excavating barbets

Abstract

The patterns and processes underlying morphological diversification are strongly influenced by the functional requirements of organisms. Bird bills are structurally and functionally diverse, and are uniquely suited to study form-function relationships. When birds use their bills to excavate nesting cavities, physical stresses experienced by the bill during nest excavation may be linked to bill shape and material composition. This relationship is affected by broad environmental factors like climate, which influences the mechanical properties of available substrates, as well as proximate factors like stress dissipation by the bill structure. This thesis explores the patterns, drivers, and biomechanical consequences of bill shape diversification in barbets, a group of nest-excavating frugivorous birds. First, we used geometric morphometrics to quantify bill shape variation and investigate the patterns and ecological drivers of shape for four components of the bill- the outer rhamphotheca and the inner bony core of the upper and lower mandibles. Next, we evaluated the excavation performance of different bill shapes and material compositions using finite element analysis and beam theory. We find that bill shape diversity has gradually accumulated over time across the two lineages, with maxillary shape driven by climate and allometry. Maxillary geometry is strongly linked to excavation performance, with deeper and wider bills exhibiting higher impact resistance and narrower bills exhibiting higher torsion resistance. In addition, the rhamphotheca and bony modules of the upper and lower mandibles have evolved as a single functional unit, and have a synergistic effect on stress dissipation. Through these findings, our integrative study sheds light on the evolution of structural and functional diversity in birds.