Abstract
Adaptation to an aquatic lifestyle occurred in the evolution of several primarily terrestrial clades of tetrapods. Among these lineages, aquatic birds’ adaptations differ in many ways from other secondarily aquatic vertebrates. As a consequence of the evolution of flight, birds with swimming and diving abilities represent unique locomotion skills and complex anatomical solutions. Here we attempt to overview some of the main aspects of avian locomotion in water and highlight the diversity of their aquatic habits and locomotion types, with the best-known extinct and extant examples. The main features that can distinguish the different groups among these swimmers and divers are their different techniques to overcome buoyancy, the transformation of wings or hind limbs into aquatic propulsive organs, and their swimming techniques besides the presence or absence of the flying and/or terrestrial abilities. Understanding how the musculoskeletal system of aquatic birds evolved to face the requirements of moving in various environments with different physical characteristics provides a good opportunity to get a better view of convergent and divergent evolution.