WHILE MOST Washington area residents huddled in chilly, homes or drafty churches Sunday morning, twoscore persons of all ages, sexes and sizes were clustered in a McDonald's parking lot listening to Hal Wierenga talk about owl pellets.

"This is from a long-eared owl," he said, holding aloft a suspicious object. "Typically it will be about the size of the last two joints of a man's thumb, and fairly regular in shape. This, now, is a pellet of the great horned; as you can see it's somewhat longer and thicker and less uniform."

He broke the long-ear pellet into fragments. There were some squeamish expressions on the faces around him until it was remembered that owl pellets issue from the front end of the bird. "You can see that the bones and teeth and claws of the prey are neatly wrapped inside the fur and feathers, which no doubt makes it a lot nicer for the bird to cough up.

"I picked up this pellet yesterday at Sandy Point (State Park) right down the road, where we're going, so we should have a pretty good chance of seeing the bird again this morning."

Wierenga, of Annapolis, is one of the thousands of dedicated amateurs in this region who spend every moment they can spare in pursuit of birds. Some are "lister," trying to check off as many as possible of the 800-odd species in North America, a sport as fiercely competitive as any. Others concentrate on particular species, or just use birding as an excuse for tramping in the great outdoors on days, like Sunday, when sensible people stay home by the fire.

His passion for and expertise about owls has made Wierenga a celebrated observer, and the response to Sunday's trip was so great the Audubon Naturalist Society had to turn most of the applicants away.

"Owls will sit pretty tight if you approach them slowly and quietly," he said. "But there is a limit. Too many people crashing through the woods on this crusted snow will drive them away."

The trip was to have started before dawn, in an attempt to call up screech, barred and horned owls, but the timing was set back to daylight because owls, unlike birders, have enough sense to hunker down when the windchill factor hovers around50.

"It has been a tough winter for owls as well as birds in general," Wierenga said. "We haven't seen anything like as many around here as normal, and it's not entirely clear why. Perhaps the unusual weather pattern has led them to range farther south or north. It may be that their reproduction was held down. Many species, almost as though in anticipation of hard times, have wild swings in their reproduction rates, so that in one year the average brood will be 10 and the next it will be half a nestling per mated pair, without any obvious reason why.

"This crusted snow certainly makes it hard on rodents, which will in turn make it hard on owls, and the colder temperatures must make it necessary for the birds to increase their food intake. But then owls will eat carrion the same as hawks and gulls, and the terrible die-off of migratory waterfowl in this region could be a plus for them. It doesn't pay to make generalizations about birds."

Trip leaders never make promises, but Wierenga was quite confident he would be able to produce at least one owl. "There's a long-ear in a pine grove here that I have seen on 15 of 18 visits," he said. "I've shown him to about 350 people over five winters, and I've spent hours right under the tree watching him. It's quite a sight to see him cough up a pellet. There's about 10 seconds of quivering and then he just leans forward and opens wide and out it comes.

"He's so tame he'll just sit and pose for you.I have gone so far as to pass a stick within two inches of his face while moving a branch that was in the way when I wanted to take a picture."

The first thicket the party explored presented a sylvan scene until the eye began to pick out the details of slaughter. Here there was a patch of blood, gone orange as it diffused through the snow crystals, where a rabbit had met his end; there was a pair of robin's wings, and yonder a patch of buff-and-gray breast feathers.

"Oh my, this must have been a chickadee," mourned Bruce Newman of Hyattsville as he picked through the tiny bits of fluff. "Everything has to eat," someone less involved with chickadees commented.

Several pines in the thicket were spotted with what birders choose to call whitewash and had owl pellets scattered below, although no owls were seen.

"Not all this savagery can be blamed on owls," Wierenga said. "There are three Cooper's hawks, several red-shouldered and some red-tailed hawks, and crows hanging around here. Together with the owls they make it a tough place for a robin. But normally is quite small and they swallow it whole or in large chunks, without scattering stuff around like this."

When an owl gets through with a mouse there is precious little left. Every fragment of flesh is dissolved by the digestive juices, and before the pellet is regurgitated it is sucked dry. "It must be as efficient a method of eating as there is in the animal kingdom," Wierenga said.

The absence of owls didn't seem to bother anybody. Along the shore by the Chesapeake Bay Bridge they already had seen the great blue heron, Canada goose, mallard, black duck (one of which put on an irresistible display of buffoonery as it skidded to a landing on the ice), gadwall, canvasback, greater and lesser scaup, buffle-head, oldsquaw, red-tailed and red-shouldered hawk, sparrow hawk, herring and ringbilled gull and a great blackbacked gull eating a duck too distant and disarranged to identify.

Also seen rafted on a rare stretch of open water were common goldeneyes, which refused to transmute themselves into Barrow's goldeneyes in spite of the best efforts of a number of people who needed the western bird for their life lists. But the birds were kind enough to put on mating displays.

The most unusual sighting of all on the Bay was a moving oil tanker (Pollutus liverius) , rumbling and screeching through the ice-choked channel opened by Coast Guard icebreakers.

Wierenga led on to the lair of his pet long-ear. On the way through the woods he and others picked off mourning dove, bobwhite, yellowshafted flicker (than which only a birder's eye is quicker), redbellied, hairy and downy woodpecker, blue jay, common crow (some tried to escalate them into fish crows), Carolina chickadee and ditto wren, mockingbird, brown thrasher, robin (with wings attached), hermit thrush, cedar waxwing, starling, myrtle warbler, house, tree, field, song and white-throated sparrows, red-winged blackbird, purple finch, American goldfinch, slate-colored junco and rock (park) pigeon.

The peregrine falcon that had been frequenting Sandy Point did not show, but some lucky few spotted the pair of bald eagles that came crusiing over, lord and lady of the skies.

Those who looked down as well as up saw the tracks of a very large white-tailed buck and those of a small doe and a fawn of the year; the frantic zigzag of a rabbit that apparently got away from a rather clumsy (weak from starvation?) fox, and boreholes in the snow where a raccoon had dug around the pine roots for creeping and crawling things.

But they did not see Wierenga's faithful long-ear and, it being lunchtime, it was decided to pack it in.

After lunch eight stalwarts trekked on to the National Arboretum, where their perseverance was rewarded with sightings of house finch and rufousided towhee and two barred owls that flushed from a copse of exotic evergreens.

At 3:30, after nine hours of effort, came the crowning touch: a male great horned owl roosting in plain sight on a pine, and a couple of hundred feet away his mate, incubating eggs on a nest in a dead stub.

"Owls nest early to get the jump on other birds,l" Wierenga said. "Half of them willbe incubating in January, and since hatching takes about 28 days, by the time the young are growing fastest and are most in need of food, the rabbits, squirrels and songbirds are producing their young, which are very convenient prey."

Wierenga was somewhat diffident about the number of owls he hadn't turned up. "I hate to drag so many people around for so long with so little result," he said."I hope they enjoyed themselves, because I enjoyed them. Birders are almost as interesting as birds, and they come in just as many shapes, sizes and personalities.

"Not finding many owls wouldn't bother me if I were alone, because I often spend all day by myself looking without finding. Just being out is satisfaction enough, and there is always something interesting out there. But when you put on an owl trip you want to show people some owls."

Nobody else seemed to mind.