Undated (AP) _ From a distance, the towering nesting platforms look like flimsy, three- legged stilts about to topple over into the marshy land below.

One of the nests is empty. But the other holds the object of this backwater search - a young osprey, its protective mother circling overhead.

Sergej Postupalsky guides his boat toward the nest, built by the Michigan Department of Natural Resources to help the comeback of this magnificent predator bird.

Postupalsky pulls out a ladder and plants it in the firm bottom of the shallow water, a flooded area several miles south of Houghton Lake.

For the osprey mother overhead, it's a familiar routine.

''His mother is an interesting female,'' Postupalsky says, as if talking about a long-lost friend. ''I banded it as an adult 18 years ago and it has returned to this specific nest each year and has outlived five 'husbands' in that span. It is an old, and productive parent.''

Postupalsky carefully scales the ladder to the nest for a look. There is but one chick, and another that didn't survive.

''Here's the other chick,'' Postupalsky says, holding the remains of another bird, now mostly feathers. ''It probably died from some unknown causes like others we have seen this year.''

He tosses the remains into the water, then gently grasps the surviving bird and takes it down into the boat for the banding process.

Like the mother osprey, it's all a familiar routine for the man known as Michigan's ''eagle man.'' Each summer since the 1960s, Postupalsky, 51, has traveled the backwaters of Michigan banding eagles, ospreys and lesser hawks and kestrals. And since then, he has watched a slow, but persistent comeback of the osprey.

At one time, these sharp-eyed predators, like eagles, were pushed to the edge of extinction due to the presence of DDT in the environment.

DDT was banned and once it left the food chain, nesting platforms were installed in many of the state's wetlands and the birds now are increasing dramatically across the state.

''Michigan has had an 'off' year for both eagles and ospreys which may be weather-related because production of both species appears to be down slightly,'' Postupalsky says. ''Still, we have encouraging news in the overall picture with birds expanding their range into areas where they have not nested in several decades.''

Where the osprey population once hovered around 80, the state has a healthy population of ospreys that continues to increase annually, mostly in several extensive wetland floodings in the north-central Lower Peninsula. Postupalsky estimates the 1986 population at about 156 nesting pairs, with about two- thirds of those concentrated in the Upper Peninsula.

''I am encouraged,'' Postupalsky said. ''They have made a comeback.''

And no one has been on hand like him to witness it.

What started as a hobby has now become something like an obsession for Postupalsky, a native of Czechoslovakia who emigrated to this country when he was 17.

It all began while he was working as a draftsman 25 years ago for Chrysler Corp. in Detroit. He joined that city's Audubon Society, and found himself increasingly drawn to the plight of the eagle. He discovered that someone was needed to monitor the bird's population, part of an effort to pinpoint the causes of its population decline.

Postupalsky volunteered. Beginning weekends, while still working at Chrysler, he made trips around the state to check the bird's nesting places and to count young.

It wasn't long before his bird work was more important to him than his job.

''It kind of got out of control,'' Postupalsky said.

He quit work, earned his bachelor's degree from Wayne State University in 1965 and his master's in zoology in 1967 from the University of Michigan.

Since then, he's worked under contract with the Michigan DNR, the position paid for by the non-game wildlife fund, which is provided through donations marked on individual state income tax returns. He spends the remainder of the year in Madison, Wis., working as a ''freelance'' biologist, doing translations of technical journals written in German or Russian.

But come each summer, Postupalsky is back in Michigan, a nomad driving the back roads around the state, staying in small-town motels and doing a job critical to the DNR's efforts to boost population of the state's predator birds.

End Adv Weekend Editions Aug. 16-17