Waxing and Waning of Wings

A major challenge to Darwinian evolution is explaining 'rudimentary' organs. This is particularly relevant to birds: rudimentary wings occur in fossils, as well as in developing, molting, and flight-impaired birds. Evidence shows that young birds flap small wings to improve locomotion and transition to flight. Although small wings also occur in adults, their potential role in locomotion is rarely considered. Here we describe the prevalence of rudimentary wings in extant birds, and how wings wax and wane on many timescales. This waxing and waning is integral to the avian clade and offers a rich arena for exploring links between form, function, performance, behavior, ecology, and evolution. Although our understanding is nascent, birds clearly show that rudimentary structures can enhance performance and survival.