Feeding ecology is the primary driver of beak shape diversification in waterfowl

Abstract

Summary The diversity of beak shapes among birds is often assumed to be largely the result of adaptations to different feeding behaviors and diets. However, this assumption has only been tested for a small subset of avian diversity, primarily within the order Passeriformes. Moreover, given the role of the beak in behaviors other than feeding and given that most previously identified beak‐feeding associations concern beak size rather than shape, it remains unclear how much of beak shape diversity is explained by feeding ecology and what functional explanations account for these differences in shape. I quantified the association between beak shape and feeding ecology for 42 species in the bird order Anseriformes (waterfowl) using 3D curvature of the upper beak collected from museum specimens and continuous dietary data compiled from the literature. I also tested whether leverage or stress resistance of the beak explains the association between beak shape and feeding ecology. Diet is strongly and significantly correlated with beak shape in waterfowl. An ancestral beak shape reconstruction and the reconstructed diet of the anseriform fossil Presbyornis both support filter‐feeding as ancestral for most waterfowl, followed by multiple, significantly convergent transitions from a duck‐like beak toward a more goose‐like beak. The evolution of a more goose‐like beak is associated with increased consumption of leaves, decreased consumption of invertebrates, and an increase in mechanical advantage of the beak. Moreover, no association was identified between size (measured as either beak size or body mass) and feeding ecology nor between size and beak shape. These results demonstrate that feeding ecology has acted as the primary selective force in the diversification of waterfowl beak shapes, including the convergent originations of geese. Thus, rapid and convergent adaptation of the beak to feeding is not limited to passerines nor is it limited to size‐correlated shape changes. The positive evolutionary correlation between mechanical advantage and herbivory shows that lever mechanics can explain the functional evolution of the kinetic upper beak in birds. These results also suggest that functions of the beak other than feeding may play a minor role in explaining overall beak shape diversity. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.12890/suppinfo is available for this article.

The evolutionary relationship among beak shape, mechanical advantage, and feeding ecology in modern birds*

“…Our results suggest that dietary transitions toward increased herbivory are correlated with evolutionary changes toward higher anterior MA, implying that herbivory imposes higher performance demands on the beak. This observation is congruent with previous ecomorphological studies on waterfowl (Olsen )…”

“…More nuanced relationships between feeding adaptation and beak shape may be operating, with variable strength, within lower taxonomic levels, to accommodate different macroevolutionary regimes and trade‐offs. For example, while a strong association between feeding ecology and beak shape characterizes the diversification patterns within waterfowl (Olsen ), skull CS, not diet, is a major driver of beak shape in diurnal raptors (Bright et al. ).…”

“…These few studies have found strong associations in several families of passerines (Gosler ; Benkman ; Price ; Peterson ; Bardwell et al. ), anseriforms (Olsen ), and a few charadriforms (Barbosa and Moreno ), but weak associations among birds of prey (Bright et al. ).…”

Feeding ecology has a stronger evolutionary influence on functional morphology than on body mass in mammals

“…A further benefit of identifying strong links between ecologies and morphologies is that these associations provide critical tools for evolutionary studies, helping researchers to reconstruct paleoecologies of fossil taxa (Meng et al. ; Olsen ; Chen et al. ), examine macroevolutionary patterns in major clades (Grossnickle and Polly ; Mitchell and Makovicky ), and reconstruct paleoenvironments (Vermillion et al.…”

Evolution of cranial shape in a continental‐scale evolutionary radiation of Australian lizards

“…Studies of bird beak shape using discrete categories have not found significant associations between diet and beak shape (e.g., Bright et al. ; Felice and Goswami ) whereas others using more sophisticated representations of diet have (Olsen ). Furthermore, sexual dimorphism in head size has been recorded in at least one species in this study (Badham ; Thompson and Withers ), and it is possible that sex also influences skull shape.…”