EVERYBODY SINGS about the bluebird, but Larry Zeleny is doing something about the gentle, beautiful and fast-disappearing American symbol of love and joy.

"When I was a boy bluebirds were as common as robins," Zeleny said as he maneuvered through early-morning traffic near his Riverdale home.

"They are open-country birds, but even city people knew them because they'd see the migrating flocks pass through. Now most people under 30 have never seen one.

"Americans have come to think of the bluebird as a mythical creature, like a cherub. It is mentioned more than any other bird in our songs and poems, but the only place they ever see one is on a valentine."

A series of man-made disasters has brought Sialia sialis, the Eastern bluebird, to such a sorry pass that now only one fugitive flash of blue is seen where 10 bluebirds once delighted the eye - and devoured crop-eating insects.

The decline began with importation of the English sparrow in 1851 and accelerated with the introduction of starlings in 1890. This ecological idiocy wrought havoc upon many American songbirds, with the bluebird perhaps the principal victim because, like the foreigners, it nests in cavities in trees and fenceposts. Chain saws, which leave fewer dead trees around, and metal fenceposts have made the housing crunch that much tighter.

The bluebird has a mild and cheerful disposition and is easily shouldered out of a nesting hole. A starling will kill nesting bluebirds. Sparrows will kill them and proceed to build a nest on their corpses.

"The male is the usual victim, because he will stay and fight," Zeleny said. "You find him under the sparrow nest with his wings spread over the babies and his head pecked all to pieces. Always the head."

Zeleny speaks with authority on the life and hard times of the bluebird, having spent much of his 73 years observing them.

The core of his campaign is establishment of nesting boxes in suitable bluebird habitat, which is nothing like so simple as it sounds. The boxes must be built just right, placed just so, and patrolled more or less regularly.

Zeleny maintains 80 boxes on a "bluebird trail" that winds for miles over the Department of Agriculture's Beltsville research center, where he was a biochemist for 25 years before retiring to fulltime bluebird farming.

He did not invent bluebird trails, as he is the first to point out, but he wrote the book on them, The Bluebird: How You Can, Helps Its Fight for Survival," published for the Audubon Naturalist Society (ANS).

The book makes easy reading, but it cost Zeleny years of patient observation, subtle experiment and upwards of 100,000 miles of patrolling his trails weekly by car and on foot.

In that time he has helped nearly 2,000 bluebirds survive to fledging (first flight). The project was adopted by ANS and the Maryland Ornithological Society, and now 83 volunteers maintain about 2,300 boxes from which more than 17,000 bluebirds have flown since 1967.

That's the good news. The bad news is that the brutal winter just past may have cut the rebounding population by worse than-hair. Attendanne is down 55 per cent in Zeleny's boxes. Reports of declines as severe as 80 per cent have come in from elsewhere.

"Th bluebird does not fly great distances between wintering and nesting grounds like other migrants," Zeleny said. "They'll winter over wherever there is an adequate supply of berries, which is what they eat after frost knocks down the insects.

"But a freezing rain or heavy snow may make it impossible for them to feed, and a bluebird's fire goes out pretty fast . . . One of our volunteers in Stafford County (Va.) found eight of them dead in one box this spring.

"When they were abundant they could over come such disasters. Theoretically a pair could raise 18 young in a season. It doesn't happen, because life is very hazardous for bird."

Especially bluebirds. Their problems include pesticides and such things as smokestacks on tobacco barns, which have trapped at least 2 million of them.

Zeleny got out to check his first box. He shook his head over the single tiny blue egg in the neat cup of dried grass.

"Same as last week," he said, making an entry on his tally sheet. "This nest seems to have been deserted. Something probably happened to one or both of the birds. I'll leave it for another week, but . . ."

As he drove to the next box a rabbit dodged into the fencerow, startling a bobolink into flight.

"If the male is killed while the female is laying - she lays one egg a day, up to about six - she'll generally leave the nest. If she's incubating or brooding she'll usually try to raise them herself, and succeed more often than not.

"If it's the second or third brood of the season, sometimes the young of the earlier broods will pitch in and help. Even a strange adult will sometimes come along and assume a full share of the job. This sort of behavior is rather rare in the animal kingdom, and it's the kind of thing that makes some of us so fond of bluebirds."

After a series of empty boxes he came to one with two tiny blue eggs. Same as last week. That's an awfully small clutch, so she probably isn't incubating, but is doesn't pay to generalize about bluebirds, they're individuals."

He checked half a dozen bare boxes and cleaned out one in which English sparrows had started a rude nest.

"If the female bluebird dies before the eggs hatch, the male must abandon them because he doesn't have the brood patch necessary to warm them.

"But if they haved hatched, he will brood them and work himself to death feeding them. When you see creatures fighting that hard for the survival of the young it gets you involved."

After another unfruitful string of boxes Zeleny came to one with five chicks a little more than a week old, snuggled rump to rump, their beaks forming the points of a star.

"I'm going to band them," he said. "I wish they were a day or two older, but by next week they'll be past 12 days. If you disturb them after that they will get so upset they'll try to fly away, and since they can't they wind up on the ground and get eaten. If you put them back in the nest they won't stay."

He lifted the chicks out one by one, giving each a gentle shake before placing it in a towel-lined shoebox. "They defecate right after you pick them up, and if you do it wrong the stuff will wind up in your hand.There's a membrane around the droppings that keeps them from soaking into the nest. The adults take them out and dispose of them, like Pampers."

Crouching over to shade the birds from the sun, Zeleny clamped an aluminum band around the left leg of each, twirling the bands to make sure they did not bind.

"Their legs shrink as they get older. They're fleshy right now, but by the time they're big enough to fly the bands will be quite loose. It doesn't seem to bother them at all."

The U.S. Fish & Wildlife Service bands ask the finder to inform the agency. Perhaps one in a hundred is reported, but every bit of data helps. Most banded bluebirds Zeleny finds are his own, so they either stayed the winter or found their way home.

Next came several boxes that had been invaded by English sparrows. "The entrance holes are no more than 1 and 9/16 inches in diameter, which is just smaller than a starling can squeeze through. Of course sparrows can use any hole a bluebird can."

Starlings and English sparrows are classified as nuisances and not protected by law. Zeleny wishes it were more widely known that starlings taste fine on toast. Sparrows nests he destroys, but he cannot bring himself to evict the field mice he often finds in his boxes. "It's the way they look up at you with those big eyes," he said.

"You might want to take these home and try them," he said, handling over several brown-streaked eggs. (Sparrow eggs taste much like any others, and hard-boil in seconds. Do not eat them in front of children.)

Down the road a piece Zeleny nit the jackpot: six healthy chicks. The parent birds, each bearing a grasshopper, lit on a fence and watched without comment as he banded their babies.

"Sometimes they'll scold or fly at you. Mostly they just wait quietly until you go away. They apparently don't notice, or mind, that the chicks have been handled and banded. It doesn't seem to be their nature to make a fuss."

The male bird sidled along the wire, one eye on Zeleny's car and the other on the nesting box. He peeked in, bobbed out to check Zeleny once more, and then went in to feed the chicks.

A car pulled up and stopped. "Any luck on this one?" the driver asked.

"A big brood," Zeleny said. "All healthy. Doing fine."

"That's wonderful," the man said.

"Why are you driving today?" Zeleny asked.

"There's a report that there's somebody wandering around with a gun, they've got the police helicopter out."

As Zeleny checked the next box, songbirds sounded form the woods on either side of the road. "The first was a yellow-throated vireo," he said. "The other one's a red-eyed vireo. I've been very lucky with my hearing. Most men my age have lost their perception of the higher frequencies, but I still can hear the birds about as well as ever."

And see them, too, at distances that astonish a less experienced birder. At one point Zeleny pointed out a flicker, an indigo bunting, a scarlet tanager and a kingbird at extreme range without missing a beat in the conversation."I've been fascinated by birds since I was a boy in Minnesota. Started banding them at 14. Met Olive (his wife) in an ornithology class at the University of Minnesota, which was a good thing, because only a birder can put up with a birder over a lifetime."

Another box ahd a single bluebird egg. "There were five two weeks ago, but they were all gone last week. It could have been a snake, they'll eat the eggs without disturbing the nest cup, but I'm inclined to think it was a two-legged predator."

Vandals rank just above raccoons as hazards to the Beltsville bluebirds. The most serious predators are sparrows, followed by snakes, which can slither up a slick metal pole in nothing flat. Over the years Zeleny has developed designs that defeat most nonhuman predators most of the time.

"Bluebirds will keep trying in spite of almost anything," he said. "If they weren't so persistent I guess they'd be extinct by now. I had a female last summer who spent three months trying to incubate an infertile clutch."

Pesticides are often implicated when birds sicken or lay nonviable eggs. "Sometimes it's pretty clear. A couple of weeks ago the University of Maryland called me over to band some nestlings. One was dead and another was dying. The nest was next to an orchard where they were spraying. Even when it doesn't kill the birds it wipes out the insects they need."

Bluebirds could live 8 to 10 years, but most do well to see three seasons. Zeleny's birds often starve when something happens to the parents.

Once he found three near the point of death and took them home in his shirt. After weeks of hand-feeding (with dog food, among other things) every 20 minutes, they took up residence in the yard, where a cat got one.

The survivors mated and ptoduced a healthy brood, giving the Zelenys rare intimacy with what must have been the last bluebirds raised within the Washington urban island. Zeleny's account of the experience makes a moving chapter of his book, buthe wouldn't want to go through it again. Sharling the life of a wild thing is a privilege for which there is a price.

The sun was high by the time Zeleny reached the site of his original nesting box, on the lawn outside the building where he spent most of his career. "Bluebirds normally shum buildings, but I put the box up when a secretary saw a pair of them hanging around. They moved right in and it has been occupied every year since, until this year. I had to replace the box, it grew so ramshackle (he's used up about 600 boxes at Beltsville), and I guess they don't like this one."

A box behind th building held four nestlings and their mother, who did not move as Zeleny lifted the lid.

"Sometimes they'll hiss and strike, but usually they are entirely passive," he said, as he slowly reached in and lifted her out. She lay still in his hand.

"Ah, she has a band," he said, squinting at the number. "I think it's one of mine, but I'll have to check."

Placed back on the nest, the bird fluffed herself over her babies. Zeleny was smiling as broadly as if she were the first bluebird he ahd ever held.

The total so far was 23 eggs and 35 nestlings, plus five chickadees and six titmice, whose perfect circle of upraised beaks look like a ring of tiny yellow flowers.

Zeleny welcomes any songbirds that use his boxes, although he confesses to some distaste for house wrens, which savage bluebirds.

The last dozen boxes, usually among his most productive, all were empty. He paused for a few minutes at the last one, mounted on a post near a pasture gate. 'This is a good location, because grazed or mown grass is low enough for them to see crawling insects easily.

"There were eggs in this one earlier, but something threw them out. I had hoped the birds would try again."

Then, as he started to drive away, a male bluebird fluttered up to the box and, after a long around, went in. "You go get mama and bring her back," Zeleny said softly.