To city dwellers, who feast their eyes on plain house sparrows and starlings, the sight of a great blue heron taking wing on the Chesapeake Bay is a fantasy in flight.

They are arguably the most regal of fliers. The great blue herons' average wingspan is six feet and their compact bodies, four feet in length, pierce the air bullet-style. Behind their eyes flash jet-black plumes.

Now is the best time to see these birds in flight, as thousands make their maiden journeys from nests at Nanjemoy in southern Maryland's Charles County to other parts of the country.

Near the banks of Hancock Creek, the young ones test wobbly wings while balancing on the edge of nests in tulip poplars 40 feet high. Adults swoop in with dinner, often crabs or striped bass. After a few weeks, the fledglings take off on their own, flapping their giant wings methodically in slow waltz time, and casting shadows like pterodactyls on the forest floor.

The kingdom of the heron spans nearly the entire coastal United States, but the Chesapeake Bay region is home to a huge population; at last count, Maryland reported 27 colonies and Virginia, 34, for a total of more than 7,000 heron pairs.

Nanjemoy is considered the largest rookery, or breeding site, on the East Coast north of Florida. Female herons choose mates each February and stay with them until the breeding season ends in July.

Because the herons are colonial birds that dwell in trees "apartment style," one catastrophe could cause extensive damage to the whole colony. "A single event, man-made or natural, in the Nanjemoy rookery would wipe out one-third of the {Maryland} population," said Glenn Therres of the Maryland Department of Natural Resources. "That's why we're starting to pay more attention to these critters."

The 288 acres that make up the Nanjemoy preserve were bought by the Nature Conservancy, a private conservation group, 10 years ago after the area was threatened by logging interests. As the first project of its Maryland chapter, the conservancy drummed up area support through the "Save a Nest Program," which became the "spark that got the conservancy going in the state," said Wayne Klockner, assistant director for the Maryland/Delaware Field Office.

Years ago, heron plumes were high-priced crowns for ladies' hats. After heron hunting was outlawed in the United States in 1918, the depleted population quickly rebounded.

Still, there are modern-day threats to the nesting environment such as logging, development and pollution, as well as nature's own logger, the beaver.

Nowhere near endangered, in fact, downright common, the great blue heron can be spotted on virtually any shoreline of the bay area waterways.

Erika Wilson, a biologist and Audubon Society naturalist, cites the size and productivity of the bay as reasons behind this population boom.

"The Chesapeake Bay is one of the largest estuaries in North America. And the general improvement in air and water quality has helped the food supply," she said.

Since heron hunting was outlawed, the biggest problem confronting the creatures is the loss of nesting habitat, Therres says. "The nature of their breeding habits is that they concentrate in small areas, versus robins, for instance, that nest all over," he said.

Maryland is doing more than paying polite attention to the birds.

This summer, the Chesapeake Bay Critical Areas Law takes effect. The statute will provide protection for the herons and Maryland's endangered species by regulating timbering, industry expansion, and development in the bay region.

In July, the rookery is nearly abandoned. Empty nests and skeletons of ill-fated baby birds that toppled from their nests mark the breeding site.

There are a few latecomers though, pairs of birds who arrived after the great influx of mid-February and are still raising their young. And there is Cal Posey.

It was Posey who made sure that the herons would have a place to which to return. He approached the Nature Conservancy with the idea that Nanjemoy was an ecologically valuable site worth preserving. Now he's the site manager for the rookery.

"I keep watch," he said, "and see that the herons aren't disturbed."

Posey, 63, has been watching the Nanjemoy herons since the mid-1940s after a nearby swamp where they were nesting became a pig farm and the birds moved en masse to the site at Nanjemoy. Every year since then, the herons have returned.

When exactly they appear is still a mystery. Although Posey can predict almost to the day when the herons will arrive, he has never actually seen them fly in.

"It's usually around Valentine's Day. One day there's nothing, and then they're everywhere. It's an explosion of birds.

"Since 1980, the population has doubled," he said. "This year there are 1,400 nests."

Posey is a firm believer in the conservancy's philosophy of hands-off preservation. Often this means ignoring hundreds of baby herons that, after falling from the nest, are abandoned by their parents and face certain death. Posey recognizes this as "part of the natural order."

One time, however, a member of a group field trip he was leading prevailed upon him to save a crippled heron. So he fed it and nursed it through the winter.

"This bird used to sit on the porch steps and wait for me to come home," Posey said. Normally herons are spooked by humans cracking twigs 40 feet below.

By spring, the wing had healed and the heron set sail for distant shores. Or so Posey thought.

"Every now and then there's a familiar honk on the road," he said. "I think it's the heron coming by to say hello."