One of the things Peter Mott's students learn in his biology class at Fieldston School is that he thinks numbers are important. He's been known to have them count the number of fallen leaves on a particular patch of ground.

For many years, Mr. Mott himself has been counting birds on the Fieldston campus. That's hardly surprising, since he is also the president of the New York Audubon Society. His observations form the basis of a recently completed scientific paper, and also of his column in the January-February issue of The Urban Audubon, the Society's newsletter.

They lead him to a surprising conclusion, one that can help illuminate this area's perennial battle to preserve green and open space and demonstrate that what's at stake are neither selfish nor parochial concerns.

As he walked to work along Riverdale Avenue one morning in May, 1985, Mr. Mott recalls in his Urban Audubon column, he heard the distinctive song of the parula warbler coming from a tree on the Fieldston campus. For 11 years thereafter, he watched the five-acre area behind the school library during the migratory season.

Between April 25 and June 5 of those years, he observed 25 species of warbler, including the mourning warbler, a somberly fledged bird spotted infrequently enough to cause John Kieran to exclaim over sightings in Van Cortlandt Park 40 years ago. Nine species have come to Fieldston every year; another six have missed only a year or two; and all of them have visited for at least two years. "If you come to visit on the busiest day of migration, day 130, you could expect to find up to eleven species," Mr. Mott reports.

The plot he surveilled constitutes no urban Eden. It's a rather shabby patch of ground concealing a small parking lot from which a rutted track leads to well-tended tennis courts and to abandoned older courts that have fallen into ruin.

The trees that dot the landscape are scruffy. Most are thin and some are twisted; they are remarkable more for what they say about nature's tenacity than its beauty. A pair has pushed through the asphalt to grow in the doubles alley of a disused tennis court. Another, struggling to find sun, has bent through a chain link fence, and has burst the links as it continued to grow.

But Beth Robinson, one of Mr. Mott's students, established the importance of these slatternly trees to the bird life they shelter and nourish. True to her teacher's methods, she counted the trees in the little woodland. There are 400 of them, and 69 percent are oaks, the warblers' favorite tree. The spent 92 percent of their time in the oaks, she found.

She also noted that different species used different portions of the trees for cover. The mourning warblers favored the underbrush, as did the yellowthroats and five other species. All but two species spent time in the suckers that are trimmed away by landscapers and home gardeners concerned with the appearance of specimen trees.

"The 'Fieldston Warblers' are a delight," Mr. Mott writes. "They please and they raise another questions. How important are these patches? How many of these pockets are there in the city? What will happen if they go?"

Can they, he wonders, be as important cumulatively as the larger oases being classified as Important Bird Areas, places like Jamaica Bay and Central Park? Should the, he asks, be identified and protected in the same way?

Isn't this just another way of sounding the same warning the Riverdale Nature Preservancy sounded when it found that if Riverdale's institutions, including Fieldston School, expanded to the maximum allowed by their acreage they would add more than 8.6 million square feet of buildings? Isn't it just another way of saying what opponents of building a massive filtration plant in the Jerome Park Reservoir have been saying?

For decades, Riverdalians have struggled to strike a balance between development and open space. For decades, their instincts have told them what Mr. Mott's study tells him: how valuable, how nourishing a patch of green is, and how much it contributes to the human endeavors in its midst.