The ospreys are back in the forest preserves around Palos Park. A pair has been resident there the past two summers, but they didn’t seem to lay any eggs in the nest they built; ospreys typically spend a few years building a nest before they use it. One of the birds I saw last Sunday was perched in a tree next to the snag that supports the nest. Back in 1854 Robert Kennicott reported that this species was known to nest in Cook County. There are no records since that time, so if these birds were to nest they would represent the return to this county of a species that has been absent for nearly a century and a half.
Ospreys would certainly be a welcome addition to the local avifauna. They are impressive creatures with a wingspan of as much as six feet. Among the birds of prey that capture live food, only eagles are larger. Their plumage is white below and brown above, and they have white heads marked with a dark stripe across the eyes.
Ospreys are fish eaters. “Fish hawk” is the most descriptive of the common names for the species. Although ospreys have gone their own evolutionary way in the direction of a specialization in catching fish, there is no doubt of their close relationship to hawks and eagles. Their scientific name, Pandion haliaetus, combines a name from Greek mythology with the Latin for sea eagle. According to some sources, Pandion was the father of Procne and Philomela. These sisters are major characters in the sort of story the founders of our civilization used to explain the world. This particular tale involves rape, mutilation, and a mother murdering her only son, cooking him, and feeding him to his father for dinner. The outcome was that Procne became a swallow (hence the genus of the purple martin, Progne), and Philomela became a nightingale. Apparently whoever named the osprey thought Pandion became a fish hawk.
Ospreys may be the most widely distributed birds in the world. They nest in North America, Europe, Asia, Australia, and probably Africa. We have no reports of nesting from South America, but our North American birds winter there, and their winter range extends as far south as Chile and Argentina.
Although ospreys got less publicity than bald eagles, they suffered as badly from the effects of DDT. The two birds have similar diets, but ospreys tend to concentrate more on live food and less on carrion. Bald eagles commonly watch ospreys hunt. If the fish hawk captures a fish, the larger bald eagle dives on the osprey and forces it to drop its prey. Often the eagle plucks the falling fish from the air. This habit of thievery was one of the objections that Benjamin Franklin raised to the selection of the bald eagle as our national bird.
Once, many years ago, I watched an osprey catch a fish at Saganashkee Slough in the Palos preserves. When I saw the bird it was flying low over the water on a line directly toward me. It was coasting with its wings held in the distinctive crook that helps birders identify this species from quite a long way off. Think of a chicken wing. It has three sections: an upper wing with a single bone, the equivalent of our upper arm; a middle wing with two bones, the equivalent of our forearm; and a small outer section that is the equivalent of our hand. Ospreys hold their upper arms level. Their forearms bend somewhat upward, and their hands bend down, creating the crook.
The coasting bird I was watching suddenly swung its feet forward, raised its wings to a high angle above its back, and dove, feet first, into the water. It rose again before its wings and head were submerged, rising on deep wing beats with a rather large fish firmly gripped in its talons. According to the books, ospreys can carry off a fish weighing up to four pounds, and this fish looked like it approached the upper limits of osprey capabilities. As the bird flew, it shifted the body of the fish until it was carrying it head foremost, tightly gripped with both feet.
The feet of ospreys have become adapted to the birds’ slippery diet. The four toes are all of equal length, and like owls–but not like other hawks and eagles–ospreys can reverse one toe. Hawks and eagles carry prey with three toes on one side and one on the other. Ospreys can carry a fish with two toes on each side. The pads on the bottoms of their feet are covered with slender, pointed protuberances called spicules, which are also useful for hanging on to something as slippery as a fish.
The nest that the Palos ospreys have built is quite large, and that too is typical of the species. The birds return to the same nest year after year, and each year they add to the structure; the biggest, oldest nests can weigh half a ton. The birds carry broken branches from the ground to the nest, and they also break off dead branches with their feet while in flight.
The usual nesting location for the species is on standing dead trees–snags–near water, but the species will quite happily use nesting platforms built by humans. Such platforms once lined the Atlantic coast, from Connecticut south. In the DDT era the platforms were almost all empty; now those that remain have begun to fill up again, and new platforms are under construction. The birds seem to tolerate considerable human presence around their nests, and some lucky shoreline property owners can watch the growth of nestlings from their living room windows.
Despite the ability of these birds to coexist with humans, I was very careful to keep my distance from the nest last Sunday. I was part of a group of eight birders, so we were in a position to create a big disturbance if we got too near. We sacrificed a really close look at the osprey to avoid bothering the birds. Think how neat it would be to come back in September and watch fledglings chasing their parents around begging for another helping of fish.
DDT was so insidious because of its ability to bioaccumulate. Way down at the bottom of the food chain, filter feeders would absorb a small amount of the chemical from the water around them. They could neither excrete the DDT nor break it down, so everything they took in stayed in their bodies. The animals that ate the filter feeders got a substantial dose of DDT with every meal, and everything they took in stayed in their bodies. At the top of the aquatic food chain, predators such as lake trout and northern pike contained concentrations of toxics thousands of times greater than what was in the water around them. And the ospreys and bald eagles were eating top predators.
DDT, like PCB, is a hormone mimic. It upsets the sexual physiology. In female birds calcium metabolism is controlled by sex hormones. DDT interferes with normal metabolism, causing birds to lay eggs with shells so thin they cannot survive the incubation process.
However, there must be more to the disappearance of ospreys than DDT and PCBs, since neither of those chemicals existed in 1855. The birds could have been driven out by disturbances along the rivers. They could have been affected by major declines in fish populations that followed heavy fishing by people and pollution of the streams with sewage and industrial waste. As hunters who need to see their prey to catch it, they would have been hurt by the heavy siltation of our rivers, as large amounts of soil washed into the water.
The birds now taking up residence in Palos are benefiting from improved water quality in the lakes scattered around the preserves and in the Des Plaines River. And they are probably helped by the fact that the Forest Preserve District and the state of Illinois are stocking fish in the lakes for the benefit of anglers. Hatchery fish are famous for being easier to catch than truly wild fish, and ospreys are likely to be as eager to grab an easy meal as the rest of us.
I am looking forward to the time when platforms in Grant Park and Montrose Harbor bring nesting ospreys to the lakefront, and we can all sit and watch the fish hawks feeding coho to their nestlings.