Morphological integration of the avian beak facilitates evolution along lines of least resistance

Innovation of the avian beak has facilitated a grand radiation of >11,000 species, with vast morphological disparity suggesting limited developmental constraints on beak diversification. We assess four macroevolutionary currencies - integration, disparity, phenotypic evolutionary rates, and ecological specialization - using 3D beak landmarks for 8,627 species mapped to a complete avian supertree with a resolved genomic backbone. We introduce a Gini coefficient-based metric of ecological specialization, measuring evolutionary time spent across trophic niches. Phylogenetic regressions show that lineages with faster phenotypic rates exhibit stronger beak integration (landmark covariation) and more generalised diets, while beak disparity declines with greater trophic specialization. These results suggest that integration facilitates, rather than constrains, phenotypic evolution, by channeling variation along lines of least resistance. Future work should explore modular structure of the bird beak, which arises from multiple genetic and developmental factors.