Large feet are beneficial for eiders Somateria mollissima

1. Many waterbirds have fully (totipalmate) or partially webbed (palmate) feet that are used for locomotion in aquatic environments. 2. If webbed feet and wings both contribute to efficient diving, we predicted a positive association between the area of webbed feet and the size of the frontal locomotor apparatus (wing area, heart mass, and breast muscle, after adjusting for any partial effects of body size). We predicted that individuals able to acquire more and better quality food due to larger webbed feet should have larger livers with higher concentrations of fat‐soluble antioxidants such as vitamin E, and invest more in immune function as reflected by the relative size of the uropygial gland than individuals with small webbed feet. 3. Here, we examine if the area of webbed feet is correlated with locomotion, diet, and body condition in a sea‐duck, the eider (Somateria mollissima). We analyzed an extensive database of 233 eiders shot in Danish waters and at Åland, Finland during winter and early spring. 4. Eiders with larger webbed feet had a larger locomotor apparatus, but did not have larger body size, they had larger uropygial glands that waterproof the plumage, they had larger beak volume and larger gizzards, and they had higher body condition. 5. These findings imply that eiders with large webbed feet benefitted in terms of locomotion, feeding, and reproduction.