My most memorable look at an osprey happened one morning in early spring at Saganashkee Slough. Saganashkee is one of the shallow lakes in the Palos Hills in southwestern Cook County. It is a pretty good sized body of water, almost three miles long east to west, and nearly a half mile at its widest point north to south.

I was standing near the west end of the slough, scanning the water with binoculars. Saganashkee is a good place to look for migrating waterfowl in early spring, and ducks were what I was hoping to find. Then I caught sight of a big bird sailing low over the water about a half mile away. It was flying almost directly toward me, so all I could see were the leading edges of its wings and the slight thickening of the outline formed by its head and body at the center of the spread wings. It was a Stealth bomber profile and very hard to read.

For just a moment, I thought it might be a great blue heron, because its wings were crooked near the tip and great blue herons sometimes soar in that position. But as I watched, it raised its wings in a sharp vee and dropped like a stone into the water. An instant later, it rose on labored wing beats, showering the water with silver droplets. A fish at least 18 inches long hung from its talons. Obviously, this was not a great blue heron.

The osprey held its catch with both feet. It kept the head of the fish pointed forward, a position that cuts down on wind resistance. With its breakfast still wriggling, the osprey flew to a tree along the south edge of the slough. It landed on a large horizontal limb, shook its feathers, looked about for a few moments, and then began to tear at the fish with its hooked beak.

One of the common names for ospreys is “fish hawk,” and my little story should tell you why. Bald eagles eat some fish and so do red-shouldered hawks, but among the world’s birds of prey, the osprey is the most specialized for a finny diet. It is a big bird. A large female–as in most raptors, females are larger than males–can weigh four pounds and boast a wingspan of up to six feet.

Ospreys can carry off fish as heavy as they are, and they prey on a full range of species, not just sluggish swimmers like carp, but trout, salmon, and pike as well. The bottoms of an osprey’s toes are covered with tiny pointed spicules that help the birds hold on to slippery prey.

Their plunges into the water are made feet first, since their feet are their weapons. They also keep their heads pointed down, presumably so they can see what they are chasing, and their wings are up out of the way in the steep vee I saw at Saganashkee. They may just skim the surface if the fish they are after swims shallow enough. But sometimes they disappear completely. Ospreys have been known to get their talons hooked into a fish that is just too big for them. The fish dives and takes the bird with it, and the osprey drowns.

In flight, an osprey shows long, broad wings with white linings, black-and-white barred flight feathers, and a dark patch at the wrist. A soaring bird will usually show the distinctive crook in the wing that I noticed at Saganashkee. The belly is pure white. The head shows a white crown and a thick dark stripe that extends from the forehead through the eye to the back of the neck.

Ospreys are distinctive enough to be grouped in a separate subfamily all their own. Some naturalists think they are more closely related to storks than to other birds of prey. The subfamily Pandioninae contains only the genus Pandion, and the genus contains only the species P. haliaetus. It is one of the world’s most widely distributed birds. It nests all over North America south of the tundra. In Eurasia, its range extends from Scandinavia and Eastern Europe all the way across northern Asia to China and Japan. It also nests on the coasts of the Mediterranean, Red Sea, and Persian Gulf, and in the Philippines, Indonesia, New Guinea, and Australia.

The North American birds do not nest south of Baja California, but they do migrate in winter all the way to southern Brazil. Robert Kennicott reported in 1854 that the bird was known to nest in Cook County, but that was about the last such report. Its current status in Illinois is as an uncommon migrant. Several birds have been reported around Chicago in the past couple of weeks; you can find them along the lakefront in both spring and fall. The first osprey I ever saw was perching in a tree next to the lagoon at Wooded Island in Jackson Park. Four were seen in one day recently at Montrose Harbor. Four birds in a day is a very high total for this species in northeastern Illinois.

Several sightings were reported in Illinois during the breeding season this year, but no nests were found. Perhaps some of these summer visitors will eventually settle down to breed. Bald eagles have returned to the state as nesting birds; we can hope that ospreys will do the same.

If they do, their nests should be easy to find. Like bald eagles, they build bulky stick nests that they reuse–and add to–every year. In natural surroundings, they prefer trees as nesting sites, but they will quite happily use structures made by humans as well. In some places, they have made pests of themselves by building in the tops of electric poles. Along the east coast, landowners used to put up poles topped by cartwheels in order to attract ospreys. The birds were regarded as picturesque, and they were also believed to drive other hawks away. Farmers hoped to scare off potential poultry eaters by bringing in these harmless fish hawks.

Unlike many hawks and eagles, ospreys are quite tolerant of each other. Where conditions are right–that is, where food is abundant–they may create nesting colonies with individual nests little more than 50 yards apart. At one time, a colony on Gardiners Island off Long Island was estimated to hold 300 nests. Gardiners Island measures about seven miles by three miles. One colony in Florida held nine nests in an area of only about 100 acres. Some nesting colonies on the east coast are as much as ten miles inland. Distance seems to mean little to these superb flyers.

A few decades ago ospreys got hit with the same poison that almost did in our other fish-eating hunters: DDT. DDT in the environment gets absorbed by animals at the bottom of the food chain, and each link in the chain increases the concentrations. Ospreys feeding on predatory fish are at the very apex of the chain, so they get the biggest dose. The DDT interferes with the production of sex hormones, and in female birds the collection of calcium for making eggshells is under hormonal control. Birds with too much DDT in their systems produce thin, weak eggshells that are deficient in calcium. The shells seldom survive incubation.

Roger Tory Peterson has written of what happened to ospreys in a colony near his home in Old Lyme, Connecticut. “There were about 20 nests in one concentration on Great Island. One season they produced six young, one year three, and one year only one. Normal success should have been between one and two young per nest.”

The total colony around Old Lyme had 150 nests in 1954. By 1960, there were 71. Only 24 remained in 1963, and by 1965, just 15. In 11 years, the colony’s nesting population had dropped 90 percent.

Like bald eagles, double-crested cormorants, and other fish eaters, ospreys have begun to make a comeback since DDT was banned in 1973. They can again be seen regularly along the mid-Atlantic coast as far north as Maine.

We need to continue to monitor this species closely, however. Other toxic substances–PCBs, toluene–could affect them. We know that bald eagles nesting along the shores of the Great Lakes have less reproductive success than birds nesting inland. Ospreys could face similar troubles.

We also need to worry about chemicals on their winter range. DDT is still widely used south of the Rio Grande, and so are other persistent pesticides. Our North American ospreys are still getting regular doses of poison with their meals for at least part of the year.