The shapes of bird beaks are highly controlled by nondietary factors

Significance We show that beak and skull shapes in birds of prey (“raptors”) are strongly coupled and largely controlled by size. This relationship means that, rather than being able to respond independently to natural selection, beak shapes are highly constrained to evolve in a particular way. The main aspects of shape variation seem to correspond with specific genes active during development. Because raptors are not each other’s closest...