"Did you hear him?" Virginia Carroll asked the group of bikers she was leading.

The deep stacoato beats, coming from somewhere high, perhaps one of the tall tulip poplars, resonated through the woods.

"That's one of our pileated woodpeckers," Mrs. Carroll said triumphantly.

Everyone looked up among the trees trying to spot the big red-crested bird, but it was apparently well hidden in the thick canopy of leaves.

The pileated woodpecker doesn't adapt very well to suburban encroachment, but in the patch of woods between Carroll's house and the Dulles Airport Access Road two miles west of Tysons Corner, at least two of these elusive birds are surviving, if not thriving.

For Carroll and her neighbors on Besley Road and Bois Avenue, the pileated woodpecker has become the colorful emblem of their battle to preserve their neighborhood, a place they call, with unabashed pride, "a very special kind of environment."

In rapidly developing Fairfax County, their environment is indeep special. The hilly woods behind their huge lots is carpeted with laurel, ferns, holly and wild hydrangeas. Oaks and tulip poplars soar more than a hundred feet above the ground, which still bears tracks of a long-ago logging operation.

There is nothing unusual about people in Fairfax battling against developers to save woods. When there are a lot of woods, as there still are in Fairfax, and a lot of developers trying to satisfy the extraordinary deman for houses, especially expensive ones, there are bound to be such conflicts.

Most of the time, the friends of birds and trees lose. They especially lose when the developer is following the county's own master plan.

The rezoning sought for the 37 acres of woods behind the homes of Mrs. Carroll and her neighbors was not only within the limits prescribed by the master plan but on the low side of the allowable density range.

The plan says the area could be developed at two to three houses an acre. The developer, a joint venture headed by Fairfax zoning lawyer Mare E. Bettius, at first proposed to build about three houses an acre, but then revised the application down to about two houses an acre.

The neighboring residents wanted the zoning kept as its present density - set before the current master plan was adopted in 1975 - of one house an acre. At this lower density, they said, their "special environment" - and the pileated woodpeckers' could be preserved.

But, as the developer's lawyer, Russell S. Rosenberger Jr., recently was able to tell the county Planning Commission: "This application is in full compliance with the criteria adopted by the Board of Supervisors."

Even the county staff, faced with the density suggested by the master plan, recommended a rezoning that would permit the developer to build about two houses an acre.

But then unusual things began to happen at the Planning Commission meeting. After listening to the residents, the developer's lawyer and county staff members, Planning Commissioners Halley A. Merrell said:

"I believe, I sincerely believe, that this parcel . . . was totally and completely overlooked by the staff and the commissioner from Centerville, Myself, and by this full commission .. . I know a member of this commission always talks about using the plan as a guide. I believe in this particular case, our plan, if we followed it blindly, would misguide us."

Merrell built his case around the credo of Fairfax's countywide master plan - that the purpose of the plan was "to preserve and enhance the quality of life" in Fairfax.

"When this plan was first drafted back in 1974," Merrell said, "one of the leading lights on the marquee was preservation of the quality of life for the citizens of this country . . . But it's a hard job, you've got a half a million people and 400 square miles to preserve the quality of life. We're trying our best. Sometimes we succeed and sometimes we don't . . . If the commission upholds my proposed action and the Board of Supervisors sustains it, we'll make another attempt to at least preserve the quality of life in you area."

But last week, when the supervisors took up the case, the struggle to preserve the quality of life encountered a formidable obstacle - the likelihood that the developer, if he had his application turned down, would sue the county.

After hearing the cold facts laid out by County Attorney F. Lee Ruck, the supervisors tried a solution that, they hoped, would give both sides at least half of what they wanted - zonging that would permit an average of about 1 3/4 houses on each acre.

First the supervisors had to rebut some of the arguments of the angry residents. Christopher Spooner had maintained there "is no way a pileated woodpecker is going to take up residence in R-17 (the zoning category sought by the developer)."

Supervisor Marie B. Travesky (R-Springfield) countered: "Pileated woodpeckers not only live in R-17 but R-12 (a denser development), and any morning you want to, you can take them off my drainpipe."

According to ornithologists, however, it is extremely unlikely that the pileated woodpecker could settle down in subdivided suburbia. Richard F. Banks, a zoologist with the Fish and Wildlife Service's bird and mammal laboratory at the Smithsonian Institution, said Mrs. Travesky's woodpecker is probably a yellow-bellied sapsucker or perhaps a flicker.