Morphological innovation and biomechanical diversity in plunge-diving birds.

Innovations in foraging behavior can drive morphological diversity by opening up new ways of interacting with the environment, or limit diversity through functional constraints associated with different foraging behaviors. Several classic examples of adaptive radiations in birds show increased variation in ecologically relevant traits. However, these cases primarily focus on geographically narrow adaptive radiations, consider only morphological evolution without a biomechanical approach, or do not investigate tradeoffs with other non-focal traits that might be affected by use of different foraging habitats. Here, we use X-ray microcomputed tomography, biomechanical modeling, and multivariate comparative methods to explore the interplay between foraging behavior and cranial morphology in kingfishers, a global radiation of birds with variable beaks and foraging behaviors, including the archetypal plunge-dive into water. Our results quantify covariation between the shape of the outer keratin covering (rhamphotheca) and the inner skeletal core of the beak, as well as highlight distinct patterns of morphospace occupation for different foraging behaviors and considerable rate variation among these skull regions. We anticipate these findings will have implications for inferring beak shapes in fossil taxa and inform biomimetic design of novel impact-reducing structures.