The evolution of morphological traits is often strongly influenced by functional and biomechanical demands. Perhaps the best example of this is the avian bill, a multifunctional appendage consisting of an inner bony core and an outer keratinous rhamphotheca, which presents a unique opportunity to study form-function relationships. Among the varied functions of the bill, certain groups of birds use their bills to excavate tree hollows for nesting, roosting, and feeding. The physical stresses experienced during this mechanically demanding task may be linked to bill shape and material properties, and also to broader factors like environmental conditions which influence the availability and mechanical properties of the substrate. Here, we examine these relationships in the frugivorous Asian and African barbets, which occupy diverse climatic regimes and excavate nest hollows in trees. Using micro-CT scans of museum specimens coupled with landmark-based geometric morphometrics, we find that bill shape diversity has accumulated gradually over time in both clades under allometric constraints, and exhibits a significant relationship with climatic variables. Secondly, using finite element analysis and beam theoretic approaches, we find that maxillary geometry trades off with excavation performance under different loading regimes. Our study thus aims for an integrative, interdisciplinary understanding of the evolution of morphological traits in birds.