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Common terns fly as buoyantly as butterflies. Their bodies rise noticeably with each beat of their wings. Hundreds of them have been passing along the lakefront over the past couple of weeks, heading south to Colombia, Brazil, and Argentina. Some will reach the Strait of Magellan.

The common is one of four species of terns seen regularly in the Chicago area. All of them nest--or have nested--around here, but nearly all the terns we see now are birds of passage who spent the summer on Lake Winnipeg or Green Bay.

Terns are very close relatives of gulls, sharing the basic color scheme of black, white, and gray, although some species have orange or red beaks. Terns almost always fly with their beaks pointed down rather than straight ahead in the manner of gulls. The beaks are straight and sharply pointed, not hooked like the bills of gulls. Many terns have black caps, and all our local species have forked tails.

So if you see a black-white-and-gray fork-tailed bird flying buoyantly along the lakefront on narrow, pointed wings with its beak pointing at the water, you are looking at a tern, not a gull.

The species you are most likely to see are common and Forster's. Unfortunately for the novice bird-watcher, these two are practically identical. It takes a very good look under very good conditions to make a positive identification. Birds do have a strong tendency to look alike, a trait that adds both charm and painful confusion to the sport of watching them.

Both of these terns are black-capped in summer plumage. The average wingspan of the common tern is 30 inches. The average wingspan of the Forster's tern is 31 inches. This is not a difference that is easy to see in the field. The National Geographic guide Birds of North America says that Forster's terns have longer bills than common terns, but I can't even detect a difference in the paintings in the book. Forster's terns' beaks are slightly paler, a little less fire-engine red, but the knowledge of this, like many birding field marks, is something you can use only after you have looked at several hundred terns of both species.

The best distinguishing mark is wing color. The wings of common terns get darker toward the tips; the wings of Forster's terns get slightly paler. Even this can be hard to see on a flying bird if light conditions are less than perfect.

The similarity of these two terns led early ornithologists to regard them as races of a single species. It wasn't until 1844 that Thomas Nuttall described Forster's tern and named it for Johann Forster, a German naturalist.

In spite of their physical similarity, they have very different ways of life. Common terns nest in huge colonies on beaches. Forster's terns nest, usually in colonies, in marshes. They nest on top of muskrat houses or among reeds or on floating mats of vegetation. Common terns are fish eaters. When they are flying buoyantly along the lakefront, their beaks are pointing down because they are searching the water for alewives. When they see one, they dive, straight down, beak first, wings partly folded. They hit the water with a splash, are completely submerged for a moment, and then bob up with a fish held crosswise in that long, red beak.

Forster's terns are mainly insect eaters who catch their meals on the fly like swallows. Patrolling the air above their home marshes, they chase down flying insects or pluck small creatures from the water's surface without wetting a feather.

Of course, now you are thinking that we have another field mark. If it dives, it must be a common tern. But no. Forster's terns can dive too, and they will as they fly along the lakefront. However, diving does separate terns from gulls. Gulls do not dive. They can reach food only if it is on the surface. They are omnivorous scavengers who can pull meat off a floating fish with their hooked beaks, but they can't catch a fish eight inches below the surface. Gulls are excellent swimmers who spend a lot of time on the water. Terns have webbed feet, but they are weak swimmers. If they aren't in the air, they will probably be perched somewhere on land.

The case of the common and Forster's terns reveals the hazards of relying solely on laboratory evidence--whether study skins or DNA prints--to demark the boundaries of species. These two are physically very similar, but their favored habitats and ways of living are quite different. It takes field study of wild, free-living birds to learn that.

Common terns are having very serious problems locating peaceful spots to settle down and raise a family. Their beaches are becoming resorts, and idiots in dune buggies roar through their colonies, crushing eggs and young. The range map in Peterson's Field Guide to the Birds shows the species declining in an area that includes the entire Great Lakes Basin and much of eastern Canada. Many long-established colonies in this vast region have been abandoned.

Our biggest tern, in fact the world's biggest tern, is the Caspian. Caspian terns are bigger than ring-billed gulls. Their wings span 50 inches, and their bodies are much stockier than the slender forms of the smaller terns. Their black caps, bright red beaks, and forked tails separate them from gulls.

As its name suggests, the Caspian is a cosmopolitan bird. The type specimen came from the Caspian Sea, but it also breeds in Green Bay and New Zealand. It dives for fish like other terns, but it also soars like a gull and sometimes flies with its beak pointed forward.

Caspian terns around Green Bay have recently had the misfortune of becoming environmental monitors, poster children for the disease of environmental degradation. Green Bay has been heavily polluted, mainly by waste from paper mills on the Fox River. The Fox carries a real witch's brew, including PCBs, and the terns are getting heavy doses of this poison.

PCBs are among the chemicals that bio-accumulate, occurring in ever larger concentrations with each step up the food chain. Fish-eating birds, as well as lake trout and salmon, are at the top of the chain. Caspian tern colonies on the bay show much larger than normal incidences of reproductive failure, and the chicks that do hatch are subject to lethal deformities.

Crossed-bill syndrome is one of these. Instead of fitting perfectly together, the upper and lower mandibles point in different directions, one off to the right, the other to the left. Adult terns feed their young by stuffing fish down their throats, so the deformed chicks can get by as long as they are in the nest. As soon as they are on their own, they die.

Our fourth and smallest tern is the black tern, an endangered species in Illinois. Black terns in breeding plumage are gray-winged and black-bodied. The only bit of white is under the tail. In fall, they molt into a winter pattern of black cap and white underparts. The fork in the tail is quite shallow, like a wedge cut from the tip.

Black terns live very much as Forster's terns do. They nest in marshes, in loose colonies, often building their nests on the roofs of muskrat houses. And they make a living by hawking insects from the air. They are fast and agile enough to chase down dragonflies.

Black terns also pluck insects from the water's surface, and especially from cattails and reeds, gleaning bugs from the leaves without ever landing.

They are a common species in suitable habitat. Almost any good-sized marsh should have lots of them. They are in the air practically all summer, flying with mothlike wing beats among the muskrat houses.

This once common species has become endangered simply because its habitat has almost disappeared. And its population levels have fallen so low that there might be no birds available to colonize new habitat even if some were created.

The black terns now passing through Chicago will leave North America entirely for the winter. Some will fly as far south as Chile. We won't see terns again until next April, when they will come north in search of nesting places. We should hope they will still find some.

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