The mechanics of diving birds and ecological implications therefore
There are two main types of diving birds: foot propelled divers, such cormorants, darters, most diving ducks, loons, grebes, the now extinct Hesperornithes and the bizarre enantiornithe Yungavolucris, and the wing propelled divers, which follow the rich tradition of aqua-flight in tetrapods, and include the iconic penguins and auks, as well as eiders (and a few other diving ducks), the extinct plotopterids, dippers, Pelecanoides petrels, and several “casual” diving birds like gannets and seagulls. It’s difficult to know exactly how and why some lineages pick one over the other – especially when some groups, like plotopterids and cormorants, are very closely related but opt for different swimming styles -, but from the looks of it, foot propelled diving seems to be most common in divers descended from terrestrial birds, while wing propelled divers seem to have evolved directly from species well adapted to extensive powered flight. In both cases, diving is a slight modification of the most instinctual locomotion style: running for terrestrial birds, flying for aerial ones.
Needless to say, there are some clear differences in the end result. Most foot propelled divers are “shore species” frequenting mostly freshwater or costoal biomes – a few cormorants and possibly Hesperornithes nonwithstanding -, while wing propelled divers do better in most marine environments, and are quite rare in freshwater biomes – aside from dippers and “casual divers” like seagulls. In addition, foot propelled divers are cosmopolitian – aside from the exclusively northern hemisphere Hesperornithes and the exclusively south american (as far as we know) Yungavolucris – while wing propelled divers are usually constrained to temperate and polar environments, being conspicuously absent from warm waters (but see below).
The reason for these differences in distribution is because both swimming styles are indeed mechanically very distinct. Wing propulsion achieves faster speeds on average than foot propulsion, while the latter allows a higher degree of manouverability over wing propulsion (Schreiber, Elizabeth A. & Burger, Joanne (2001) Biology of Marine Birds). This means that foot propelled divers do better in more constrained shallow water environments, where quick manouvers are more vital to hunting, while wing propelled divers thrive in open sea environments, where chasing down prey is the main hunting style. In addition, studies seem to suggest that wing propelled hunting is most efficient at water temperatures below 15ºC; warmer water temperatures allow the usually ectothermic fish to swim faster, out swimming their avian predators. This is why most penguins and auks are forced to inhabit cold waters, and even tropical members of both clades are restricted to the Humbolt current areas in the eastern Pacific.
And then came the exceptions
But evolution managed to fuck up with these results. If wing propulsion is inefficient at warm water temperatures, why do giant penguins and plotopterids occur in tropical waters in the mid-Cenozoic? Similarly, why is the ostensibly wing propelled Pinguinus found in both subtropical and freshwater biomes? And, likewise, if they occupied similarly warm waters, why didn’t they expand to the south or north? Why do large Hesperornithes occur in the vicinity of aqua-flying polycotylids, if either lifestyle would mean a triumph of either group in a particular marine biome?
The answer seems to be size. Larger animals can produce more raw power than smaller ones, and as such, speed and manouverability curves are less extreme than in the smaller birds; this rings especially true for the large Hesperornithes, which seem quite designed for speed. The restrictions in range are more difficult to solve, especially when in both cases the only competitors were whales, which didn’t become a significant threat until the Miocene.
In regards to the Great Auks, the fact that they are thought to have been deep diving specialists might have allowed them to expand their range, as mesopelagic/benthic biomes are both colder than surface waters and have a larger variety of food items like cephalopods. Pinguinus alfrednewtoni seems to have had a more diverse range than Pinguinus impennis, with specimens found in both freshwater environments and in subtropical areas, though the Great Auk’s range in Florida and on Madeira seems to suggest that it periodically could reside in subtropical environments on specific occasions.
Auks and plotopterids, while definitely wing propelled, are/were for the most part more manouverable than penguins, having larger and wider feet, so their competitive disadvantages are/were also not as extreme.