John Page Williams is one of those rare birds who has found his perfect spot in life.

Williams is field trip director for the Chesapeake Bay Foundation taking groups on excursions into the mudflats of the Potomac, the grass beds of the Bay, up the creeks and down the rivers to see what's growing, what's flying, what's swimming and what's simply sitting there.

He's young and strong, virtues that keep him at the head of the pack when the paddling and slogging become tough. And he's handsome as a pirate, which doesn't do him a bit of harm with the matrons who make up a good part of his clientele.

Most of all, John Page Williams loves to talk.

He can talk endlessly about saw grass and reeds, about mummychaugs and shad, beaver dams, tidal flats, wildfowl diets, heat, cold, shorebirds or anything else that sticks its head up long enough to earn a lecture.

Last weekend he led a party of 17 into the fascinating backwaters of the Potomac at Mason Neck, a 1,300-acre national wildlife refuge 20 miles south of the District near Lorton. Mason Neck was founded eight years ago to provide habitat for four eagles (that's right, four). It was a choice between that and another Reston, this one to be called King's Landing, and for once the conservationists won out.

The eagles are down to two now, but the vast marshes and woods of the refuge provide a home for thousands of wild animals and birds, and with migrating waterfowl stopping this is the height of the season.

It didn't take Williams long to set up his first lecture site on a barren lump of low, muddy ground.

Barren, that is, to the unpracticed eye.

"See this?" he asked the assembled canoeists, snapping off a dried-out stalk of nothing. "This is wild rice, and it's one of the things that makes this place so good for waterfowl."

Williams explained that the Chesapeake and its tributaries are one of the three best wild rice-producing areas in the nation. Minnesota and South Carolina are the others.

Why is that important? Wild rice is high in both protein and carbohydrates and the plants have a good yield of grain. As the weather gets colder ducks and other waterfowls need tremendous infusions of high-value grain to keep from freezing.

"They have a much higher surface to total body mass ratio than the bigger animals and a body temperature of more than 100 degrees. Even with their oily, down coats they need a lot of food to keep body heat up," Williams said.

Other stalks of "nothing" turned out to be tidemarsh water hemp, tearthumb and smartweed, some of the two dozen identified before the day was done.

The birders had a good day, as well, although a party of 18 in nine canoes is not an ideal approach to sneaking up on wary wild ducks.

But as they paddled through the shallow mudflats the weekend naturalists kicked up huge flocks of mallards and black ducks, hooded mergansers, a blue-winged teal, wood ducks, and saw from afar magnificent red-tailed hawks, mightly blue heron and pileated woodpeckers.

The eagles? No soap. Assistant refuge manager Steve Wunderley said they are nesting this year off nearby Pohick Bay, but there's hope that they will move into the refuge before spring.

To that end the park, unfortunately, will be closed from Dec. 1 to March 31. "We don't want people wandering through the woods when they're picking out their nest. They may not come here at all, but if they do we want them to go wherever they want. We don't want to lose those birds," said Wunderley.

Canoeists with a yen to visit the flats can still get in by putting in at Pohick Bay and paddling the three miles north to the little inlets and creeks that wind back into the refuge.

For those interested in a guided tour with Williams, he'll he moving around the Bay and its tributaries with his mobile canoe rack and his easy patter for some time.

Call the Chesapeake Bay Foundation in Annapolis for details. The numbers are 301 - 268-8816 or 301 - 269-0481. Or write to P. O. Box 1709, Annapolis 21404.

If you phone, pick a time when you have a few minutes. You can't tell who will answer the phone, and John Page Williams does love to talk.