An Ecomorphological Analysis of Diet and Keratinous Beak Structure in the Nearctic-Neotropical Raptor Genus Buteogallus

Skeletal integration and allometry in raptors restrict beak morphology to a size axis and preclude shape changes in response to diet. Despite this, three dietary ecomorphs in the Buteogallus clade could be differentiated in PCA morphospace by keratinous beak structures. Five linear distance and two curvature measurements were taken on the rhamphothecae of preserved museum skins (n = 67). The outer and inner rhinotheca curvatures and cere displayed greater magnitudes and influence with diet specialists, except for insectivores. Dietary generalists were associated with intermediate magnitudes and influence, and insectivores were associated with the weakest. Intra- and interspecific morphological differences varied by species, rhinotheca surface, degree of rhinotheca curvature, and cere length. The three keratinous beak structures were significantly related to diet. Results suggest that the cere and curved surfaces of the rhinotheca are independent evolutionary platforms that can respond to selective dietary pressures alone or as a complex. Of the 22% beak variance not directly attributable to allometry and integration, the cere and rhinotheca curvatures explained 16%. Proportionally, those structures were associated with 73% of the unconstrained shape variance in the clade. The formation of keratinous structures over the underlying bone in avian beaks may be examples of evolutionary tissue decoupling. Keratinization may have released the avian beak from skull and maxillary constraints, facilitating novel morphological changes in response to diet and feeding pressures