Aspects of diversity, paleobiology, and morphology of wing-propelled diving birds

• Wing-propelled diving in birds is more common than generally considered. • Trade-off in flying wing-propelled divers is not in wing size itself. • Body mass of flying wing-propelled divers can exceed 1 kg. • Multiple evolutionary pathways exist for specialized wing-propelled divers. Bird wings are typically regarded as an apparatus for aerial flight, but many birds are known to use their wings to propel themselves in the water. Although this mode of locomotion, the wing-propelled diving, has attained much attention from ornithologists and paleornithologists, systematic understanding of its diversity has been lacking. This paper attempts to provide an overview on this topic, gleaning information from disparate bodies of literature. Despite the common perception that wing-propelled diving is a relatively specialized mode of locomotion, it has been documented in a number of casual divers as well as many birds usually considered as specialized foot-propelled divers. The fossil record of wing-propelled divers reveals a previously unrecognized morphological diversity in these birds, although difficulty remains on how to reliably infer the presence of this locomotor mode in extinct taxa. Recent anatomical studies have indicated that evolutionary pathways toward wing-propelled diving can be more complicated and diverse than one would assume from the stereotypic idea of evolutionary convergence. Future investigations equipped with well-resolved phylogeny and sophisticated analytical techniques will undoubtedly shed light into more nuanced aspects of the evolution of this mode of locomotion.