Wing Morphology Varies with Age but not Migratory Habit in American Dippers

Abstract

We investigated variation in morphology of American Dippers (Cinclus mexicanus) in the Chilliwack River watershed, British Columbia in relation to gender, age and migratory habit. Male dippers had linear dimensions that were 2–9% longer and, on average, were 16% heavier than females. Adults (AHY) were the same structural size as yearlings (HY). Yearlings, however, had shorter and more rounded wings than adults providing support for the hypothesis that an increased vulnerability to predation may lead to selection for traits that improve take-off performance and maneuverability. Yearlings also had shorter tails suggesting other selective pressures shape tail morphology. Dippers in this population may be sedentary or migrate short distances to breed at higher elevations. We found no evidence that wing or tail morphology varied with migratory habit or that sedentary dippers, that have higher reproductive success, are larger or heavier than migrants.