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ELIZABETH TERWILLIGER -- a naturalist and conservationist -- is Pied Piper to thousands of San Francisco Bay Area children each year. Called Mrs. T. by both youngsters and adults, she's a tanned and youthful lady with a wide-open smile. Much of the time she's clad in no-label jeans, a plaid flannel shirt, and a broad-brimmed straw hat tied under her chin. If not bobbing along trails with groups of enchanted children, Mrs. T. can be found darting through town in a vintage Volkswagen bus filled with the stuffed cormorants, egrets, and pintails (all natural casualties) that she uses in her bird-in-the-hand approach to nature education.

Thirty years ago, Mrs. Terwilliger began taking children and their families on hiking, biking, and canoeing trips through the marshes and beaches of Marin County, Calif. She is at it still -- six days a week, 48 weeks each year. It is her belief that children made aware of the environment at an early age will grow up to be environmentally responsible adults.

Mrs. T. received President Reagan's Volunteer Action Award in May of 1984, and this year the Terwilliger Nature Education Center, founded to perpetuate her philosophy, celebrates its 10th anniversary.

``What's important,'' says Mrs. T., who has fought to create open spaces and keep them accessible to the public, ``is where are people going to go with their children?'' When her own son and daughter were small, she found no place for them to play. So she organized a group of parents to build a playground. Years later, when her children were unable to bike safely to school, she began a 20-year fight for community bicycle paths. Today, there are bike paths not only all over Marin County, but across the G olden Gate Bridge as well, largely because of to her efforts.

Mrs. T. was also instrumental in persuading each of the towns surrounding Richardson Bay (an arm of San Francisco Bay) to build community docks.

This project had small beginnings that snowballed. Mrs. T. was slated to teach mothers to canoe in one community. But the only way to get the canoes into the water was to portage through stretches of mud; more of the lessons were spent in slush than water. It then occurred to Mrs. T. to invite the city council for a little canoeing trip. After the council's outing, plans began to roll for the docks.

Mrs. T. is a strong advocate of canoeing. ``You can't play soccer or baseball all your life,'' she says. ``But you can canoe all your life, alone or with your family. It's quiet, it doesn't use any oil, and you can learn about tides, marshes, and birds.''

In discussing community projects and environmental battles past, the seasoned conservationist confides, ``You never know what you can do until you try.'' One time she sat in the front row -- with a stuffed egret and heron on each side (again both natural casualties) -- of a city council meeting called to discuss filling in a salt marsh. Destroy the marsh, she told the developers, and you will kill all these birds. The developers didn't develop the marsh. And Mrs. Terwilliger believes she helped turn the

tide on this project.

Another time she brought a classful of children to Marin Civic Center to see the swallows up in the rafters whose nests were to be swept away in the interests of clean walkways. The broom operation was shelved, and the resettling of the birds was handled later in a more gentle fashion.

Strolling along the shores of Richardson Bay, she points to a sign prohibiting bicycling and restricting access after 6 p.m. on the path that runs between the shore and the homes alongside. She believes this particular shoreline, like all of nature, should belong to all children for all time.

``It's a constant challenge,'' she says. ``While you're sleeping, someone is planning.''

Mrs. T.'s background is as varied as the wildlife she studies. A descendant of Kit Carson and James Fenimore Cooper, she grew up on a Hawaiian sugar plantation. She earned a degree in home economics at the University of Hawaii and a master's in institutional management from Columbia. She then went on to Stanford to study nursing. But it was while hiking in the back country of Yosemite and biking through the hills of New England that the thrill of outdoor discovery made its lasting imprint upon her.

Later, when she realized there were no nature programs available in California for her children, she volunteered to lead classes on field trips. Her program grew, encompassing walks with preschoolers and mothers and Saturday family adventures. Her goal, then as now, is teaching children ``to look and see and wonder,'' she says, ``not to break and destroy.''

``Why save the environment for the children if you're not going to educate them?'' she asks. To educate them to love the environment, not to be afraid of the out-of-doors, is what's important, she says. ``Knowledge dispels fear.''

Mrs. T. uses an approach that nudges children's creativity while actively engaging their minds and bodies. ``Look at the colors,'' she says, sitting on the beach of the Richardson Bay Wildlife Sanctuary, a 700-acre wildlife refuge she was instrumental in saving and which is now Audubon's Western regional headquarters. ``Use all of your senses. Feel it, smell it, see it, taste it, hear it.'' Her voice softens and quickens. ``Which way is the wind blowing? Listen. Can you see it?''

``It's constant play-acting,'' she says, smiling, and then she demonstrates the dignified carriage of a pintail duck as it struts.

In 1969, Joan Linn Bekins (who began hiking with Mrs. T. 18 years ago) started recruiting ``nature guides'' to expand Mrs. T.'s program. Ten years ago, Mrs. Bekins and her husband, Donald, set up the nonprofit Terwilliger Nature Education Center to carry on Mrs. T.'s teachings in a more permanent way.

Begun in a tiny office in the Bekinses' Belvedere home, the center now has an annual operating budget of $190,000. One of the largest expenses is taxidermy, and for this the center uses only natural casualties donated by nature lovers. Mrs. T.'s husband, Calvin, a retired orthopedic surgeon who has always supported her conservation efforts, is now learning taxidermy.

The center is currently housed in part of a vacant public school at Corte Madera, a convenient location, since it is near one of Mrs. T.'s favorite field sites, Ring Mountain. Here, she and her young charges explore for evidence of Miwok Indians and delight in finding the Tiburon Marisopa Lily, which was discovered in 1971 and is found nowhere else in the world.

The center, which sports a mini-museum, a resource center, and three nature-equipped vans, has also produced several award-winning films that introduce Mrs. T. to more than 1 million schoolchildren across the nation each year. The films are distributed without charge by Chevron.

Last year, Mrs. Terwilliger proved to be a favorite at the White House luncheon where she received her Volunteer Action Award. She was introduced as the only award-winner who picked up litter on the White House lawn before the awards ceremony, and by the time lunch was over she had everyone in the audience, including President Reagan, imitating flight patterns of birds with waving arms.

But now the hubbub of Washington is far behind, and Mrs. T. takes up her regular strolls at Richardson Bay. As she walks, she picks up the yellow Styrofoam cooler of birdseed that she leaves stashed on the beach and then scatters seed along the tideline. It's her hope that the birds and ducks will come close enough for the children to see and identify them.

``I want the children to really know what a pintail looks like,'' she says.