For the past 20 years Jackson Abbott, who studies battlefield explosives, mine detectors and other instruments of war for the U.S. Army Corps of Engineers, has been laboring to keep the American bald eagle alive and free.

When he's not in his office in Ft. Belvoir, the 58-year-old, amateur naturalist is usually looking for eagles, counting them, charting their nest, observing them or totting up statistics on their beleaguered progeny. This year, he reports has been the best nesting season for the eagles on record.

At a bouyant, 16, Abbott spiked his way 80 feet up to a tulip maple tree to take a picture of eagle eggs in a nest. As a middle-aged man he crashed one helicopter in the process of taking movies of eagles, and had other copter perilously dive-bombed by an irate nesting female.

He has seen the wonder of the eagle's courtship flight, when the huge birds tumble hundreds of feet through the air together, locked in each other's talons, screaming at the sky.

And he's had his migration data on eagles disappear when the wily birds simply tore off their carefully placed leg bands.

Like hundreds of other Washingtonians for whom nature is less than a living but somehow more than a career, he has watched his bird work absorbed his weekends, soak up his annual leave, involve his family and interfere with his job.

But there are many rewards. This month, Abbott, whose Chesapeake Bay area survey team in the 1950s and 1960s helped document the environmental peril to the bald eagle, sound the alarm against pollution, and launch the Endangered Species Act of 1974, reports that the national symbol is propsering as never before in recent years. He published his report in connection with the U.S. Fish and Wildlife Service, the Audubon Naturalist Society, the National Wildlife Federation, the states of Maryland, Virginia and Delaware and several other agencies.

In general, the report shows, there are more eagles present in the tristate area, more eagles nesting, more eggs being successfully hatched and fewer nests being abandoned than at any time in the past 10 years.

There were 79 active nests found (up from 73 last year), and another 10 pair of adult eagles seen without nests (up from 7 pair last year) and 69 eaglets hatched during the summer, up from 39 last year.

Abbott knows all this because he and his team members have seen it. For most of the first six months of every year, they are flying about looking for eagle nests, plotting them accurately on maps and sending ground teams to compile data on the nests, the eagles in them and the immediate environment.

"The nest are usually about 6 1/2 or 7 feet across and 3 or 4 feet deep (in sticks)," Abbott said. "They usually choose the three-pronged crotch in the highest tree around" - often a loblolly pine or tulip poplar - "and often go back to the same nest or a previously abandoned eagle's nest year after year . . .

"We have one nest in Dorchester County (Md.) that's been in use since the turn of the century. It's 8 feet across and some 20 feet thick . . . they keep adding sticks year after year. It's in a huge old cedar tree that must be 200 years old."

About 3 feet in from the edge of the nest, Abbott said, the bald eagles put a carpet of marsh grass that they pull up in clumps (it looks orange when dried in the sun") and with which they cover the eggs whenever they have to leave the nest.

In the center they made a cup shaped depression in which they usually lay three eggs.

Eagle survival, however, is a fragile thing. Unlike smaller birds that may hatch quickly, grow into adulthood in a matter of weeks and breed within a year, bald eagles take a month or more to hatch, require three months of feeding before learning to fly and rarely breed before they are 4 or 5 years old.

More than half of all bald eagles born die the first year, many of those from the sharp bills of their more aggressive siblings. Those that survive remain vulnerable to pesticides, trigger-happy hunters and the intrusions of man.

The bald eagle population in Virginia, Abbott said, remains especially imperiled, Along the James River, where 13 pairs of eagles were sighted in 1964, the population nosedived to four in 1968, three in 1970, one in 1974 and none since then. "Pesticide contamination in the river and its fish population is the proven cause of this population extermination," Abbott said.

Eagles are primarily fish eaters, as similating pesticides like DDT, which in turn have been assimilated by the fish on which they feed.

Most choose nest sites in remote wooded areas near large bodies of water. Some eagles, however, prefer to eat turtles, snakes, or eels. Many feed on carion at least part of the time, and Abbott knows one pair of eagles on the Eastern Shore with a taste for chicken.

He knows all this because his survey workers checked below the nest for food fragments to learn what the eagles are eating upstairs.

A conversation with Abbott, in fact, ends up being mostly praise for his fellow eagle surveyors who include fish and game workers from state agencies, federal biologists and many peple who just care about eagles. "They're the people who really do the work," Abbott said. "That one climb up a tree when I was a noy scared me to death. I haven't done that since. But they go up to nests all the time."

Abbott was asked by the Audubon Naturalist Society in 1957 to take on the eagle survey ("everyone felt there were fewer eagles around but we had to document it and we don't know why.") He had six volunteers - fellow birders "whom I knew could tell the difference between an eagle and a crow."

Since then his team has grown, especially since passage of the Endangered Species Act, to about 20, including highly skilled biologists like Stanley Wiemeyer from the U.S. Fish and Wildlife Service's Patuxent Wildlife Research Center.

Wiemeyer has been studying levels of pesticides and other pollutants in eagle eggs and trying to do something about it. When Abbott's team finds an eagle pair that has tried unsuccessfully to hatch young for several years, surveyors climbed to the nest in question and steal an egg for Wiemeyer to analyze.

If he finds lethal chemical levels in the eggs, the surveyors can steal a couple of live eggs from several eagle pairs Wiemeyer keep in captivity, and transport them to the nest site in a styrofoam-packed attache case warmed by hot-water bottles.

The surveyors then climb the nesting tree, hoist up the attache case and switch eggs on the parent birds, who often then raise the young successfully.

All that might seem like a lot of trouble for a bird Benjamin Franklin judged inferior to the turkey as a potential national symbol. Abbott says Franklin's judgement was colored by storied that the eagle is a thief, living off fish he steals from ospreys. Abbott won't say they don't steal from ospreys; only he's seen a lot of eagle activity and never seen that. But he thinks Franklin was wrong.

"I think the eagle is an excellent national symbol," he said. "They are strong but inoffensive. They don't usually attack unless attacked themselves. They can be clumsy at times . . . not built for maneuverability and speed like a falcom, but beautiful. And they are exemplary parents, sticking together for life through thick and thin. I think the country could use a lot more of that philosophy nowadays."

But eagles to Abbott are just part of the wider natural world he says he cares about equally.

"You know, ever since I was a bou I wanted to be a naturalist. My father was an illustrator and taught me a real fascination with birds and wildlife. I never really wanted anything else.

"But World War II came along when I was in Swarthmore and I joined the army. Afterwards I wanted to go back to college and study ornithology at Cornell, but they had a three-year waiting list. So I decided to make the Army my career and be a free-lance naturalist whenever I could. Twenty years ago they asked me to do eagles, and ever since then that just seems to be most of what I've done."