Justin Schwemmer, 10, remembers finding the impression of an owl's wings in the snow, and the tiny footprints of a mouse nearby.

"The trail of the mouse ended right where the wing prints were," says his mother, Sharon, an avid bird-watcher.

Danny Schwemmer, 7, enjoys identifying the birds that visit his family's many feeders -- it's like piecing together a puzzle.

And the youngest Schwemmer, 2 1/2 -year-old Marshall, plays peek-a-boo with the woodpeckers that feed in his backyard on a homemade mixture of peanut butter, corn meal, flour and shortening. He points to the "tee," or tree, where his avian playmates come to feed.

The Schwemmer home, on 2 wooded acres west of Hampstead, is a stage for many of nature's vignettes. The backyard adventures with birds and other creatures are second nature for the boys, like digging in a sandbox or diving into a pile of fallen leaves.

For the Schwemmers, some experiences are more than fleeting. For weeks last year, the family watched a pair of great-horned owls -- powerful, nocturnal predators -- raise young in a nest just 100 feet from the house.

The family's practice of feeding birds, putting up nest boxes and keeping track of migratory visitors is a window on a world of endless discoveries.

And their hobby is one shared in varying degrees by thousands young and old. The cost can be minimal and the rewards priceless.

You can get started as a backyard bird watcher for less than $10, with a board tacked to a window sill to serve as a feeder and a bag of mixed seed from the grocery store. Binoculars, if necessary, can cost as little as $25 at discount stores.

If you don't want to bother with making your own platform, try buying a small, inexpensive feeder that can be stuck to a window with suction cups. Or simply spread seed on a tree stump and the ground.

The birds don't care whether your feeder is an Escort or a Cadillac, as long as there is food. All the better if you have a hanging feeder or one fixed to a window, and seed spread on or near the ground. Goldfinches, Carolina chickadees, woodpeckers and other species prefer the former. White-throated sparrows, cardinals, mourning doves and other species prefer the latter.

Mrs. Schwemmer, 35, was first bitten by the bird-watching bug when she was in elementary school. She has a vivid memory of her sense of accomplishment when she was first able to use a small pocket field guide to identify a bird on a feeder at her grandparents' summer home. "I ran to my mother and said the whole name: 'rufous-sided towhee.' "

She became more serious when she and her husband, Geary, a scientist at NASA's Goddard Space Flight Center in Greenbelt, took a field guide to birds while on a trip out West in 1977.

For some people, backyard discoveries lead to questions -- and an understanding of how people influence the well-being of birds and other critters, and vice-versa.

How has backyard feeding expanded the range of cardinals and other birds? What role do birds play in keeping insects in check or spreading seeds of many native plants? How has the importation of starlings and house sparrows from Europe affected populations of native birds of suburbia?

"I like to study animals that are getting extinct, especially birds," says Justin.

And the discoveries can be just as thrilling if you live in the Carroll County countryside or in downtown Baltimore, like 74-year-old Peggy Bohanan.

"Anything can show up in the city," says Mrs. Bohanan, who has been bird-watching for about 30 years. She has compiled a "yard list" of 118 bird species that have come to feed during the winter, rest during migration or are seen flying overhead.

Her 15-by-30-foot yard, as well as several other small yards behind neighboring row houses, provide an island of maples, hollies, dogwoods and other plant species in a sea of concrete.

For some, an awareness of birds in their backyards leads to spring walks with more experienced members of the Maryland Ornithological Society along the shores of Lake Roland just north of the city, or elsewhere. It also may lead to participation in annual counts of birds in winter or spring.

If the birding obsession takes hold, it can take you to strange places in search of rarities, as it did the hundreds of people who flocked to Back River Waste Water Treatment plant near Essex in March 1990 for a once-in-a-lifetime look at a Ross' gull, a bird never before seen in Maryland. The closest nesting spot for the species is Churchill, Manitoba, along Canada's Hudson Bay. It breeds more commonly in Siberia.

Bird study, either through a kitchen window or in the wind-swept marshes of Chesapeake Bay, feeds a $1 billion industry of selling field guides, binoculars, seed and other paraphernalia. The U.S. Fish and Wildlife Service estimates that more than 82 million Americans feed wild birds.

Some people combine bird study and gardening, by cultivating berry-producing species, flowering plants and cover to attract birds.

While bird-watching doesn't require a huge investment to get started, there are plenty of resources if you want to splurge.

"We've gotten a lot of people started. That's very rewarding to us," says Barbara Davis, who, with her husband Phil, owners of the Wild Bird Center in Columbia.

The store, in operation for 1 1/2 years, is one of a half-dozen sister franchises in Maryland, most of them in the Washington metropolitan area.

The Davises stock the full range of bird-feeding and -watching supplies and equipment.

The Davises sell feed for 29 cents a pound for a standard mix or up to $1.69 a pound for thistle, a favorite of certain finches.

Feeders sell from $3 for a small plastic hanging model up to $100 for the "Viewmaster," an aquarium-like device with finished wood trim that bolts into a window frame. "It almost looks like a piece of furniture," says Ms. Davis.

Much of the same feeding supplies can be found at Southern States stores or Frank's Nursery and Crafts outlets.

"People like the fact that they are helping birds. A lot of birds die in winter," says Rob Mardiney, education director at Irvine Natural Science Center, on the grounds of St. Timothy's School on Greenspring Avenue in Stevenson.

For the past 17 years, the center, a private, non-profit organization, has been offering children's nature programs and classes on birds and other creatures and selling nature study supplies and accessories.

Government-run nature centers, such as the one operated at Oregon Ridge by Baltimore County, offer similar education programs.

Irvine's sales of birdseed, feeders, nature books and other items totaled $82,000 in the fiscal year ending June 30. Profits are pumped back into education programs.

Unlike most science disciplines, bird study is dependent on the observations of amateurs. Since birds are so mobile, amateurs are the eyes and ears of scientists trying to keep tabs on the health of birds.

That has spawned a number of cooperative efforts, including Christmas bird counts sponsored by the National Audubon Society or Project FeederWatch sponsored by Cornell University's Laboratory of Ornithology and Long Point Bird Observatory in Ontario. (To learn more about Project FeederWatch, write the Cornell Laboratory of Ornithology, 159 Sapsucker Woods Road, Ithaca, N.Y. 14850, or call [607] 254-2414.)

So much isn't known about the habits of birds, Mr. Mardiney says, that even a passive observer can contribute valuable information.

Or you can just relax and watch the show.

Birds in Baltimore

These are birds that may appear in the back yards of the Baltimore region, though their occurrence depends on the season and on the habitat:

Song sparrow

House wren

White-throated sparrow

Carolina wren

Dark-eyed junco

White-breasted nuthatch

Rufous-sided towhee

Tufted titmouse

American goldfinch

Carolina chickadee

House finch

American crow

Northern cardinal

Blue jay

Common grackle

Purple martin

Brown-headed cowbird

Barn swallow

Red-winged blackbird

Eastern wood-pewee

House sparrow

Eastern phoebe

Yellow-rumped warbler

Least flycatcher

Common yellowthroat

Great-crested flycatcher

Black-and-white warbler

Eastern kingbird

Red-eye vireo

Hairy woodpecker

White-eyed vireo

Downy woodpecker

European starling

Red-bellied woodpecker

Blue-gray gnatcatcher

Common flicker

Golden-crowned kinglet

Ruby-throated hummingbird

Ruby-crowned kinglet

Chimney swift

Eastern bluebird

Eastern screech owl

Wood thrush

Rock dove

American robin

Mourning dove

Northern mockingbird

American kestrel

Gray catbird

Killdeer

Brown thrasher

American woodcock

Source: "Birds of North America," by Chandler S. Robbins, Bertel Bruun and Herbert S. Zim, and "Field Guide to the Birds of North America," The National Geographic Society.

More bird-watching information

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Other sources of information on birds and birdwatching:

MARYLAND ORNITHOLOGICAL SOCIETY CONTACTS:

Anne Arundel: Allan Havry.. . (410) 757-3523

Baltimore-Baltimore County: Shirley Geddes. .(410) 377-8462

Carroll: Bill Kulp. .. (410) 775-2264

Harford: William McIntosh. 410) 734-6517

Howard: Martha Chestem. ...(410) 730-1527

FIELD GUIDES, REFERENCES:

*"A Field Guide to the Birds," by Roger Tory Peterson, 1980, Houghton Mifflin, $15.95. A well-known and trusty guide to identification of birds in eastern and central North America.

*"Birds of North America," by Chandler S. Robbins, Bertel Bruun and Herbert S. Zim, 1985, Golden Press, $10.95. Recommended as the easiest, most comprehensive guide to birds throughout North America.

*"Field Guide to the Birds of North America," The National Geographic Society, 1987, $21.95. Another comprehensive guide.

*"Birds," by Herbert S. Zim and Ira N. Gabrielson, 1987, Golden Press, $4.95. Recommended as a good, pocket guide for beginners to learn the common birds.

*"Familiar Birds of North America," of the Audubon Society Pocket Guides series, 1986, Alfred A. Knopf, $6.95. Also recommended as a good, inexpensive guide for beginners.

*"The Bird Feeder Book," by Donald and Lillian Stokes, 1987, Little, Brown and Co., $10.95. Recommended as a good starter book for families and beginners.

*"The Audubon Society Guide to Attracting Birds," by Stephen W. Kress, 1985, Charles Scribner's Sons, $24.95. A comprehensive text. Not recommended for beginners.

*"Garden Birds. How to Attract Birds to Your Garden," by Noble S. Proctor, 1986, Rodale Press, $17.95. Espouses a "bird-based" approach to gardening and includes ways to build feeders and houses.