Many kinds of birds dive for a living. Not from a diving board, but from the air or the water surface. Some of these birds fly over the water, see potential prey, and dive from the air to attempt capture. These birds are called plunge divers. Gulls plunge into the water to about their own body length and retrieve prey from near the surface. Terns, some pelicans, tropicbirds, and boobies dive from above the surface, the first three penetrating farther into the water column than the gulls do. Boobies (and their close relatives the gannets) penetrate well below the surface and actively chase fish underwater.
Many other birds dive from the water surface, including loons, grebes, cormorants, alcids (puffins and their relatives), diving-petrels, some shearwaters, coots, and many ducks. In this discussion let’s look at this group, so-called surface divers.
All diving birds have feet that are modified for swimming at the surface, and these same feet serve them well for underwater propulsion. The typical swimming foot is that of a duck, with the three forward toes joined by webs. The hind toe is much reduced. Among the diving birds, these webbed feet also characterize loons, gulls, terns, shearwaters, diving-petrels, and alcids (this last group lacks a hind toe).
A variation on this is to have the hind toe lengthened and all four toes connected by webs. This characterizes cormorants, pelicans, boobies, and tropicbirds. Finally, instead of webs, some birds have each of the toes, including the hind one, provided with large, flat lobes that are equally efficient in propelling the bird forward. Divers with lobed toes include grebes and coots.
Surface divers vary quite a bit in how they dive and how they locomote under water. Most of them just put their head down, slide under the surface, and propel themselves downward. Although the feet stroke alternately while swimming on the surface, when underwater they stroke in unison, like a pair of oars, and they are held out to either side. This must use different sets of muscles than those used for swimming on the surface. Smaller birds such as grebes, coots, and small cormorants may jump up into the air to enter the water with more momentum.
One group of birds exhibits a real variation in this theme. The alcids are wing-propelled surface divers, using their wings to swim just as if they were flying underwater. As they go below the surface, you can see their wings already open as their means of propulsion. In the guillemots, which feed on bottom fish, the feet are used along with the wings, but in murres and puffins and others that feed on midwater fish and krill, they are held behind and not used at all. The southern-hemisphere diving-petrels, related to true petrels, also use this method of propulsion, as do penguins. Not able to fly, penguins are the most highly modified birds for underwater living.
Scoters and some other sea ducks such as Long-tailed Ducks open their wings as they dive, easily seen. They may use them as diving planes underwater or may actually flap them just as alcids do. For the most part, diving birds surface just by stopping their underwater propulsion and, like a cork, popping back up to the surface. They go headfirst, the most streamlined way to go. Alcids sometimes swim actively upward and penguins always do.
Still photos of birds swimming underwater are very hard to come by, although YouTube and other such online video sites show a variety of birds swimming underwater, very often misidentified (grebes and alcids called “ducks”). Notwithstanding these few videos, we have much to learn about how diving birds forage underwater.