Aquaflying vs Foot Propelled Swimming
With inflexible torsos and short tails, birds are forced to engage in two main forms of underwater locomotion: aquaflying or foot propulsion. There are variations, mostly in the latter (compare the “frog-like” motions of grebes and loons to vertical strokes of cormorants, for instance), but ultimately specialised diving birds end up in two molds, the flipper winged “penguin like” forms, or the hindlimbs-at-the-end-of-the-body “loon like” forms. There isn’t much margin of overlap between the two, with nearly all aquatic birds known either opting for one method or the other. Either you develop strong oar-like wings, or you don’t and let them degenerate in favour of stronger or otherwise modified hindlimbs.
That is conventional wisdom. As it turns out, many birds previously thought to be exclusively foot-propellers actually do engage frequently in aquaflying, using it almost as common and effectively as their “conventional” swimming. Most notable among these are diving ducks and cormorants, traditionally thought to be outright incapable of wing propulsion (Upstroke Thrust, Drag Effects, and Stroke-glide Cycles in Wing-propelled Swimming by Birds), but clearly very much capable of doing such.
Indeed, one notable lineage of aquafying birds, Plotopteridae, may have evolved from foot propelled divers, being closely related to modern cormorants and darters, and possibly retained it quite late in their evolutionary history.
The differences in effect between aquaflying and foot propulsion are rather clear: aquaflying has been proven to ensure faster speeds, while foot propulsion results in higher manouverability (Schreiber, Elizabeth A. & Burger, Joanne (2001) Biology of Marine Birds). Tellingly, aquaflying is most prevalent in oceanic pursuit predators like penguins, auks, petrels and seagulls, while foot propulsion is common among freshwater and coastoal birds (grebes, loons, ducks, cormorants, darters, finfoots). There are however obvious exceptions to the rule, like the freshwater dippers and the (largely) marine Hesperornithes.
In the aforementioned “foot propeller turned into aquaflyer” cases, it appears to occur primarily in marine settings, in which foot propulsion becomes less beneficial than wing propulsion. In ducks, the dive is initiated by aquaflying, and only engaging in foot propulsion when it reaches it’s feeding grounds in the ocean bottom. Most likely, aquaflying speeds up the process of descent, indeed confirming that it is a faster method of locomotion than foot propulsion on average. In the case of the cormorants, it seems to have started in the same way, using aquaflying to reach the ocean bottom faster, but since these birds are fish predators a continuously fast speed is necessary, so the animal only seems to resume to foot propulsion if the prey is stactic. From this point on, foot propulsion is best done with altogether, the animal achieving more success by continuing to rely on it’s wings.
Most freshwater birds do not have this pressure, simply needing to round up fish against trapping environments, so aquaflying seems less favoured than sheer manouverability. However, dippers and similar birds do need the sheer wing muscle power in order to compensate for their small size and higher bouyancy, so they became aquaflyers in the first place. Generally speaking, foot propulsion seems to correlate with lack of speciation, with only loons, grebes and Hesperornithes seemingly having become more specialised in it than switching to aquaflying altogether.
For now, it is too early to establish a consistent pattern to the evolution of aquatic locomotion in birds, but it is clear that there is less of a dichotomy than previously thought