Power Cascade Morphogenesis of Bird Beaks and the Generation of Exceptions

ABSTRACT Developmental processes underpinning beak morphogenesis have been suggested to govern the evolutionary diversity of bird beak shape. The beaks of most adult birds have been found to follow the power cascade, which describes the log–log linear relationship between the expansion of the beak radius and distance from the beak tip. We compared ontogenetic beak shape of species from six phylogenetically and morphologically distinct bird orders that follow (Palaeognathae, Galliformes, and Estrildidae) or do not follow (Anseriformes and Pelecaniformes) the power cascade growth model as adults. Geometric morphometrics was also used to examine how beak morphogenesis deviates from the power cascade model. Bird species with power cascade beaks in adulthood (Australian zebra finch Taeniopygia castanotis ; red junglefowl Gallus gallus ; and southern cassowary Casuarius casuarius ) exhibited a consistent pattern of growth. In contrast, species with elongate and/or compound beaks (royal spoonbill Platalea regia ; Australian white ibis Threskiornis molucca ; Pacific black duck Anas superciliosa ; and magpie goose Anseranas semipalmata ) did not follow the power cascade rule at almost all beak growth stages. However, in juvenile royal spoonbills, a more ossified internal core bony element was found to develop first and initially follow the power cascade ( R 2 > 0.90, p ‐values < 0.001). This core structure may form first, growing according to the power cascade acting as a foundation from which the lateral expansion builds and breaks the model. By using the power cascade with geometric morphometrics to assess intraspecific beak shape variation from embryo to adulthood we demonstrate how a rule of growth may govern the development of many bird beaks and how evolution of exceptions to the rule may be generated.