Sherry Sheppard says you can always tell a bird watcher. "They all have the same look, the same clothes, the same warm hats, the same equipment. That's one right there."

She pointed out a young man in dowdy brown slacks walking out the breakwater that shields this resort town from the sea. He carried binoculars and a cased telescope and wore a purposeful look. At the end of the pile of stones he bumped into Sherry's husband, Jay.

It was the Christmas bird count, 1981, and bands of bundled-up Auduboners were running into each other all around Ocean City, the best bird-watching place within driving distance of bird-happy Washington.

"You have three different habitats here," explained Jay Sheppard, who was directing a party of five birders covering one of 10 official subsections for the Ocean City count. "There's seashore, then marsh and then woods as you move inland." Ergo, many different kinds of birds.

In fact, Ocean City almost always has the highest bird species count of any of the 21 reporting areas in Maryland. And Sheppard's annual assignment is the best subsection in Ocean City--the heart of downtown where there are sea birds, house birds, woods birds and marsh birds.

The Sheppards and Ray and Gretchen Stanton took a day of leave from government jobs last week to check for bird species. They were wandering around well before dawn and still tallying long after dark, along with 30 or so other bird-lovers who make the Ocean City count their informal annual convention.

The purpose of the counts, which are conducted at more than 1,300 stations across the country, is to find out which bird species are rising, which are falling and which are turning up in unexpected places. The dream of each participant is to find something really unusual.

For Sheppard that chance came early. He was scanning the turbulent channel from the breakwater through a 40-power telescope. In the midst of a flock of sea gulls was one that stood out. It had smoky, dark underwings. The top of its wings were pearly grey. Sheppard recognized it immediately.

"Little gull!" he exulted, explaining quickly that this small European sea gull had only recently found its way across the Atlantic, and a sighting in 1971 at Ocean City had caused a major stir.

Bird watching, like most erudite pastimes, turns out to be less complicated than it might seem. I had it in mind that a birding expedition meant wandering out into the woods and being expected to recognize from a massive wellspring of innate knowledge anything that flew into view.

But Sheppard, in his 10th year of counting at Ocean City, had a long list of birds he expected to see and simply checked them off as he went along.

His first check mark was for purple sandpipers, which spend the winter dodging the spray on the breakwater and feeding among the seaweed. The sandpipers were in a large flock; they darted out from under foot as Sheppard strode by.

"They're extremely confiding," he said. "Just go slowly and you can walk right up to them." Which he did.

He spent an hour scanning the rocks of the jetty across the channel for a rare harlequin duck, but came up with only common and whitewinged scoters, red-throated and common loons, mergansers, a grebe or two and various gulls.

Then it was off by van to downtown, where house finches, starlings, red-winged blackbirds and sparrows abound. Then back to the backside of Sinepuxent Bay for a look across the low-tide mud flats at huge rafts of Canada geese, brant, whistling swans, Bonaparte's gulls, black-bellied plovers (which have white bellies, inexplicably), terns, more sandpipers, dowitchers.

The second-floor porch of a motel closed for the winter provided a platform from which to view a protected cove, and on the pilings of an old fishing pier perched a trio of ruddy turnstones. Check them off.

Across the Rte. 50 Bridge in a freshwater pond sat a huge flock of canvasback, black and mallard ducks.

And so it went, back and forth across town from first light to last light, checking off species as they hove into view, estimating numbers, hunting for rarities. Elsewhere in the 15-mile-wide Ocean City subsection, parties were following similar programs, darting from viewing station to viewing station, building up their lists.

At day's end they met for dinner, a cold, weary, sun-washed crowd. Their avuncular leader, Chandler Robbins, co-author of "The Golden Guide to Birds of North America," demanded tallies.

The party leaders gathered around and Robbins called off each species. "Black-crowned night heron," said Robbins. Dead silence.

"Come on, people. Someone, please?"

Black-crowned night heron was a 1981 failure. The big success was the sighting of a western kingbird by Peter Pyle and Sam Droege, the first such sighting ever in the Ocean City count. In all, 141 species were counted, about average. Sheppard's crew had 41 species, below his normal total.

Next summer the Christmas counts from all across the country will be published in book form, providing this year's edition in the continuing record of changes in bird life from 1900, when the first Christmas count was held.

Also, the names of participating bird watchers will be published, providing the only record of these selfless souls who give a day of their holiday time for the benefit of their feathered friends.

The birds get in for free. To defray publishing costs, each watcher pays $1.50 to get his name in, which strikes Sherry Sheppard as the ultimate injustice.