Trends of avian locomotion in water – an overview of swimming styles

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.

Specialized diving traits in the generalist morphology of Fulica (Aves, Rallidae)

“…Surface swimming is common in many aquatic birds, including ducks, cormorants, loons, grebes, gulls, pelicans, gannets, phalaropes, among others 1 . Many of these birds are also extremely adapted for foot-propelled diving, like loons, cormorants, grebes, and many ducks 1 , 2 .…”

“…Diving is a widespread skill among tetrapods that can be performed in various scenarios such as escaping from a predator or acquiring food 1 . In birds, underwater propulsion can be facilitated by either wings (wing-propelled divers like penguins and alcids) or legs (foot-propelled divers like cormorants or ducks) 1 . The body shape, pelvis, limbs, digits, and their associated musculature are quite distinctive in divers 2 .…”

“…Many birds, like ducks, phalaropes and most foot-propelled diving birds, are buoyant swimmers 1 . In birds that dive, this is because its plumage remains dry after diving 10 .…”

“…This is because they have “partially wettable” plumage 10 . Both can feed by picking items from the surface of the water, but only the former can upend their bodies and feed by dabbling 1 .…”

Aspects of cranial adaptation in foot-propelled diving birds – foraging and visual fields of some piscivorous species

“…The more advanced foot-propelled divers are characterised by an elongated body, caudally positioned hind limbs, with powerful pelvic and leg musculature to make them enable to pursuit hunt their underwater prey (Kristoffersen 2001, Clifton & Biewener 2018, Segesdi & Pecsics 2022.…”