Scaup were my introduction to the true terrors of birding. For those who do not follow the sport, I should explain that scaup are ducks. Diving ducks, to be more precise.

Diving ducks hunt for food well underwater, propelling themselves through the depths with their feet. They are to be distinguished from dabbling ducks, such as mallards, who just tip themselves over and reach their necks down as far as they can while their tails wave in the breeze. Dabblers can feed only in very shallow water. Scaup dive 20 feet or more looking for aquatic insects, snails, small clams, crustaceans, tadpoles, and pretty much anything else slow enough to catch and small enough to swallow. They also eat seeds and other parts of various aquatic plants.

The problem with scaup is that there are two different kinds, and they look very much alike. In A Field Guide to the Birds, Roger Tory Peterson treats them with his usual confidence. The male lesser scaup, he says, is black at both ends and white in the middle as it sits on the water. Its bill is blue, its head is somewhat pointed and glossed with dull purple, and its flanks and back are finely barred with black. The greater scaup has a rounder head glossed with dull green, and its flanks are whiter.

The females of the two species are brown with a white ring at the base of the bill. They are virtually identical when seen on the water. However, both sexes of the two species can be identified in flight by looking at the white stripe near the trailing edge of the wing. In the lesser scaup, the stripe shows only in the secondary feathers. In the greater scaup, the stripe extends into the primaries.

If you are not into the topography of bird wings, I should explain that the secondary feathers are attached to the second joint of the wing. Think of a chicken wing. The second joint is the part with two bones in it. The primaries are attached to the third joint. On a chicken wing this is the short, pointed tip of the wing, the part with no meat on it.

Of course chickens are not really great flyers. On a duck this third joint is longer than the other two parts of the wing put together. But it still doesn’t have any meat on it.

Greater scaup are also bigger than lesser scaup, but the size difference is quite small. The biggest lesser scaup are actually bigger than the smallest greater scaup, so size is not a good way to tell the two species apart.

When I first started birding, I was prepared by Peterson’s confident tone to name every scaup I saw, but real-life scaup are not quite as cooperative as the paintings in the field guides. To start with, most of the scaup we see around here are on Lake Michigan or on one of the larger inland lakes such as Lake Calumet or McGinnis Slough. This time of year–late October through November is the peak season for scaup–you usually get waves on large lakes. Birds sitting on the water appear as they rise to the crest of a passing wave and then vanish as they sink into the next trough. You only get a passing look, a glance that doesn’t last more than a second.

And then there is the weather. On dull overcast days with a mist rising from the water, you can’t see the iridescence that gives the green or purple gloss. With bad light and a short look, you probably won’t be able to figure out the shape of the head either, especially if the bird is far from shore–as they often are.

Of course if the birds fly, you do have that wing stripe to look at– but even that is hard to see. The next time a pair of mallards fly over, notice how fast they flap their wings. And mallards are dabbling ducks. Experienced birders can tell dabbling ducks from diving ducks just by noting the speed of their wing beats. Diving ducks flap their wings much faster than dabbling ducks. So fast, in fact, that measuring the length of the white wing stripe becomes very difficult. It is so difficult that I usually can’t do it unless I see the birds coming in for a landing and gliding on extended wings.

So most of the time the best I can do with scaup is to note the sighting of a “scaup species,” which is the honest and honorable thing to do, but very frustrating. I’m out to identify birds, and it bugs me when I can’t do it.

Ducks are famous for their loose morals. In today’s repressive sexual climate, it would be impossible for any duck to get elected to public office, even though they might do a much better job than some of the bozos we have in there now. Drakes will apparently try to mount anything that swims, and the females are just as randy. Obvious hybrids of two different species are seen regularly. Lesser scaup have been known to breed with wood ducks, American wigeons, redheads, ring-necked ducks, European pochards, tufted ducks, and canvasbacks. The amours of the greater scaup are, if anything, even more varied.

But despite all this rampant promiscuity, there has never been a recorded instance of a greater scaup-lesser scaup hybrid. You would think that birds this much alike would get it on regularly.

The best guess as to why they don’t is that they don’t live in the same places. Greater scaup are circumpolar. They nest all the way around the world in the far north, choosing lakes or ponds on the tundra for their homes. Lesser scaup are strictly New World birds whose favorite nesting sites are ponds in the northern forests. They also nest in pothole lakes on the prairies, some as far south as northern Iowa. So when breeding season rolls around and the hormones begin to flow, the two species are nowhere near each other.

Biologists who study the question of how species originate talk about isolating mechanisms, factors that separate different populations of the same species. In time, these isolating mechanisms can turn two populations of the same species into two distinct species.

Geography is the most obvious isolating mechanism. The scrub jay, a relative of the blue jay, lives in the western U.S. and in Florida. Between west Texas and Florida, a distance of nearly a thousand miles, there are no scrub jays. Apparently, climate change in the past several thousand years eliminated all the suitable scrub-jay habitat in Alabama, Mississippi, and Louisiana, leaving the Florida birds all by themselves.

At this point nobody thinks the Florida scrub jays have become a separate species from the western scrub jays, but the split could occur in the future.

Eastern and western flickers used to be considered separate species. The eastern populations of these woodpeckers are noticeably different from the western birds. The wing feathers of eastern flickers are a bright golden yellow on their undersurfaces. Western birds have red undersurfaces.

The Great Plains used to keep these populations apart, but after Europeans settled the plains and began to plant trees, eastern and western birds met and began to interbreed. The young born to these unions were intermediate in coloring and quite able to breed on their own, so ornithologists decided to lump the two former species into one.

Habitat choice and behavior are also isolating mechanisms. Changes in courtship rituals or differences in the timing of mating may prevent individuals of the same species from breeding together. In time these separated populations could develop into different species like the greater and lesser scaup.

Living things diversify. We change, develop new forms, produce new possibilities. One of the arguments for halting the destruction of rare and endangered species is that extinction kills the future, erases possibilities.

If you want to see diversification in action, you can find both greater and lesser scaup almost anywhere along the lakefront between now and the end of the year. Sometimes they sit on the water in large flocks called rafts. A raft of several thousand–identified as lesser scaup–showed up ten days ago off Zion. Flocks of as many as 5,000 have been reported in the past.

Most of these birds will be gone by the end of December. Both species winter mainly on salt water, with the lesser generally the more southerly of the two. Some lesser scaup winter in the West Indies and northern South America.

There are lots of other waterfowl to be seen along the lakefront now. You can find goldeneyes, canvasbacks, redheads, mergansers, and others. Loons are passing through now too, along with various gulls. For birders, the lakefront is more interesting now than it is in July.