In bluebird box 172, the nestlings were nearly bald, their few wispy gray feathers belying the incandescent blue they will become in several weeks. Still too young to open their eyes, they quivered at the sudden splash of sunlight as Lawrence Zeleny pried open the top of the box.

"Two days old at the most," he said of the four birds, each no larger than a walnut. "They weren't here Sunday. They were just four little eggs then."

At 86, Zeleny has been an ally of the imperiled bluebird for nearly three-quarters of a century.

Every week from February through September, drizzle or wilting sun, he faithfully examines the 60 bluebird boxes along a meandering 10-mile route through the Department of Agriculture's National Research Center in Beltsville. He began this bluebird trail in 1967 as the bird's numbers dwindled to a handful in the Washington area.

For Zeleny and the 5,000 members of the North American Bluebird Society, which he founded 12 years ago, bluebirds represent a sort of ornithological pinnacle.

"The bluebird is one of the very best loved birds in North America. They're really very charming creatures," Zeleny said. "They have become a kind of symbol of love and happiness and all nice things like that. It's a shame most people see them only on valentines."

The society has no exact figures on the number of bluebirds, and it is very difficult to get even rough statistics. But Todd Culver, the education specialist for Cornell University's Lab of Ornithology, said that before the turn of the century bluebirds "in many ways were as common as robins." The population is estimated to have dropped by 90 percent.

In the summer, bluebirds have two to three broods of about four nestlings each. Zeleny estimates 100 to 200 birds hatch on his trail each year. Although no longer found inside the Beltway, the birds are making a tenuous comeback on outlying golf courses and rural, grassy parts of Howard and Montgomery counties, said Chuck Dupree, treasurer and one of the founding members of the North American Bluebird Society.

The shy, mellifluous bird is known as a cavity dweller, preferring to hole up inside a rotting fence post or tree trunk rather than nest in the open. But as wooden fence posts have given way to metal and as trees have been axed to make way for development, the bluebird has had a tough time finding a home.

Added to that, bluebirds have been affected by the influx of house sparrows and starlings, what Zeleny calls foreign birds, which immigrated from Europe in the mid to late 1800s. The more aggressive newcomers drive the reserved bluebirds from their nests, appropriating them for their own broods.

Zeleny claims his affinity for birds began in his native Minnesota when he was 14 months old. While he was recovering from a long illness, his mother would park his baby buggy near a bird bath every day.

"The birds would bathe and drink and splash around. They were just fascinating and delightful. I remember this, believe it or not. It must be my first memory," he said.

Zeleny moved to University Park in the 1930s when he started a job as a chemist for the Agricultural Research Center. He built his trail's first bluebird house outside his office in 1965, and the box is still there even though he has long retired.

Every Sunday morning just after dawn he drives from box to box in his car with a license plate that reads "Sialia," the scientific name for the bluebird. He estimates he has made the trip more than a thousand times, checking for new eggs or hatchlings or predation.

On a recent tour he pulled a mauled nest of dry grass from the box, gestured to the smashed blue eggs on the ground and hypothesized that raccoons had raided the nest. With luck on its side, a bluebird might live to be 8 or 10 years old, but most only make it to their second or third years.

"They really messed things up," he said of the raccoons. "It doesn't do much good to get upset about it. The best thing you can do is prevent it from happening again."

Careful monitoring and special raccoon-deflecting devices attached to the boxes can cut down on these incidences, he said.

So far, this year has proved to be a bad one for breeding, with fewer birds than in recent memory, Dupree and Zeleny said. Dupree speculated that the winter's wild temperature swings cut down on the number of insects available for the bluebirds to eat.

Bluebird society members put up houses in their yards and construct and monitor bluebird trails across the country for the three kinds of bluebirds that live in the United States. The Washington area is home to the eastern bluebird; farther west are the mountain and western bluebirds, which have subtly different markings.

Zeleny gives talks and answers correspondence for the bluebird society. He recalls a woman from Alabama who wrote to say her husband was very depressed after retiring, rarely leaving the house. But after discovering bluebirds, he went out and built his own trail.

"At first my only thought was to helping the bluebirds," Zeleny said. "But in the last few years I realized you're not only helping the birds, but the birds are helping an awful lot of people enjoy life a little more."