Bill shape imposes biomechanical tradeoffs in cavity-excavating birds.

Organisms are subject to physical forces that influence morphological evolution. Birds use their bills as implements to perform various functions, each exerting unique physical demands. When excavating cavities, bird bills must resist a range of mechanical stresses to prevent fracture. However, the contribution of bill geometry and material composition to excavation stress resistance remains poorly understood. Here, we study the biomechanical consequences of bill diversification in the cavity-excavating palaeotropical barbets. Using finite-element models and beam theory, we compare excavation performance for two loading regimes experienced by barbet bills during cavity excavation: impact and torsion. We find that deeper and wider maxillae perform better for impact loads than for torsional loads, with the converse for narrower maxillae. This results in tradeoffs between impact and torsion resistance imposed by bill geometry. Analytical beam models validate this prediction, showing that this relationship holds even when maxillae are simplified to solid elliptical beams. Finally, we find that composite bill structures broadly exhibit lower stresses than homogeneous structures of the same geometry, indicating a functional synergy between the keratinous rhamphotheca and bony layers of the bill. Overall, our findings demonstrate the strong link between morphological evolution, behaviour and functional performance in organisms.