A few years ago I had a student, Brett, who had played baseball while he was in high school. One day in lab the students and I were chatting about nothing in particular when the conversation turned to the difficulty of memorizing the scientific names of all the animals they were studying. We got into one of those debates about the usefulness of common names as opposed to scientific names, and I got on my soapbox to deliver my usual sermon: common names are fine, if you are talking with non-scientists and as long as the names are unambiguous, but for scientific communication and to avoid confusion and ambiguity you need to use an organism's scientific name. Also, taxa that have been well studied for decades or centuries, such as birds and flowers, often have common names that are widely accepted and used by both scientists and naturalists. This evolved into a discussion of bird-watching and how birders have developed a sort of shorthand for birds' common names; RTHA for red-tailed hawk (Buteo jamaicensis), for example.
At this point Brett chimed in with a bit of wisdom imparted by one of his high school baseball coaches, who said, "All you need to know to identify birds is whether or not it has webbed feet. If it has webbed feet, it's a duck! And if it doesn't have webbed feet, it's a pigeon!" I must say, as far as methods for distinguishing different groups of birds, I have heard worse. The possession of webbed feet at least has a functional significance, and it's usually easy enough to see a bird's feet, or at least to infer the presence or absence of webbing by observing the bird in its habitat.
My own proficiency at IDing birds is sketchy at best. I'm pretty good with the birds that I see all the time in my backyard and canyon, and I can get most other sightings down to major group, but there are some types that I will probably always find difficult. Gulls and the wading shorebirds, for example. Gulls are notoriously problematic because there are many species and they go through three or four juvenile stages before attaining adult plumage. Wading shorebirds (sanderlings, and whatnot) all look alike to me, and absolute size differences are hard to discern when dozens or hundreds of birds running up and down the beach are about the same size.
One bird that I can easily identify based on its silhouette, is a cormorant. They are related to pelicans and have the same gular pouch under the throat that they use to catch fish, but are their bodies are much more streamlined. Unlike pelicans, which dive from the air to catch fish, cormorants are pursuit divers, using their webbed feet to swim after fish below the surface. These webbed feet are located at the posterior end of the body, where they are well positioned for propulsion under water (think about where a submarine's propeller is located). Having their feet at the back end of the body gives cormorants a more upright stance on land compared to pelicans, whose feet are positioned towards the middle of the body and thus carry themselves along a more horizontal axis.
Clearly, despite their webbed feet, cormorants are not ducks. However, like ducks they do spend most of their time on or in the water. Cormorants are unusual for aquatic birds in that they don't have oil in their feathers. You've heard the phrase "Like water off a duck's back", right? It means not being affected by external events, instead letting them roll off and away the way that water beads and falls off a duck's plumage. The saying is true because ducks and other waterfowl do indeed have a coat of oil in their feathers. In fact, most birds have feathers that are water-repellent to some degree. The oil keeps water from penetrating through the feathers and chilling the body. It also provides additional buoyancy. When you see a bird preening, part of what it is doing is distributing the oils over the feathers in an even coat.
Not having oiled feathers, cormorants soon become waterlogged, which enables them to stay underwater and swim efficiently below the surface. Unfortunately, getting soaked to the skin means the cormorants are susceptible to hypothermia. When they have finished feeding, they need prepare their feathers before they can make any prolonged flights. You will often see cormorants perched on rocks or cliff ledges, basking with their backs to the sun and wings outstretched. They have to do this to dry their wings and warm up their bodies before they can fly. Their dark coloration absorbs heat quickly and speeds up the drying process.
Even though they don't have oiled feathers, cormorants do spend a lot of time preening. They use their beak to smooth feathers and make sure they lie properly on the body. Usually they pay special attention to the wing feathers, as the proper condition and alignment of these feathers makes flight possible.
Another characteristic that makes cormorants different from ducks is their solid bones. Almost all of the flying birds have hollow bones, to lighten the load they have to carry through the air. Flight is a very energetically expensive endeavor, and over millennia the hollow skeleton has evolved to make it slightly less so. Penguins, of course, do not fly in air, but their swimming motion is essentially underwater flight. They have solid bones, to provide weight and counteract the positive buoyancy generated by their blubber and oiled feathers. Like their flightless tuxedo-wearing relatives, cormorants also have solid bones, to help keep them underwater as they pursue fish.
The hitch in this plan for cormorants, however, is that they do fly. Cormorants travel through the air and hunt for prey in cold water. They certainly aren't the only birds with this combination of habits; there are tern species, for example, that migrate thousands of miles and feed by plunge diving. But cormorants, being pursuit divers, spend more time underwater than most other flying birds. They have had to evolve a combination of adaptations for flight (flight feathers, wings long enough to enable flying) and adaptations for swimming underwater (legs at the back of the body, lack of oil in feathers, and dense bones). Natural selection is often about just this sort of compromise. An organism doesn't have to be perfect to be fit for its environment, but it does have to be good enough. And when an animal spends time in both air and water, it has to be good enough in two environments. Cormorants, traveling through air and hunting in water, manage to be successful at both and thus persist.