When snow blankets your backyard birds' natural food supply, your first impulse may be to fill a feeder with prepared birdseed from a nearby store.

But few of us know exactly what mix is best for wild birds, particularly during tough winter months.

Two years ago, Aelred D. Geis of the Patuxent Wildlife Research Center supplied some of the answers in a study for the U.S. Fish and Wildlife Service. His findings were based on 179,000 observations of feeding birds in the Baltimore-Washington corridor.

"White proso millet and black oil-type sunflower seeds are eagerly taken," he reported. "Yet, such common ingredients of commercial mixes as flax, canary and rape (a type of mustard seed) are rarely eaten by birds.

Moreover, Geis found, birds will choose sunflower seeds and white proso millet even over such tasty items as wheat, sorghum, cracked corn, rice, milo and oats. And one common ingredient in commercial mixes -- peanut hearts -- tends to draw nuisance starlings and grackles rather than songbirds.

Even if your commercial mix contains generous amounts of sunflower seeds, it pays to look beneath the surface -- literally. Break open a seed and see if it contains anything worth eating. A recent check in our household revealed dried, empty seed pods.

You often can find white proso millet and the oily, black sunflower seeds at a farm cooperative or quality nursery. If you can't find them anywhere, try wheat, cracked corn and oats -- which are generally cheaper and serve very nicely as a substitute. Bulk bags are available at farm cooperatives such as Southern States, Piney Farms and the Maryland Tobacco Growers Association.

But seed alone won't bring the birds flocking. It's just as important to have a proper feeder. The National Wildlife Federation offers good advice on types of feeders, the birds they attract and the food that works best.

Ground-feeding stations -- a patch of yard or simply an inverted hubcap or garbage-can lid -- attract jays, sparrows, mourning doves, chickadees, juncos, house finches, even pheasants in rural counties. Scatter sunflower seeds, cracked corn (if you don't mind blackbirds) and peanuts.

Tabletop or window feeders will attract cardinals, goldfinches, grosbeaks, chickadees, purple finches, jays and mockingbirds. Sunflower seeds, shelled unsalted peanuts, various seeds, raisins and currants turn the trick.

For high post or hanging feeders, you can use milk containers, thoroughly washed bleach bottles and, of course, birdhouses. Cardinals, chickadees, titmice, nuthatches, finches, redpolls and pine siskins will be regular visitors. They love thistle, sunflower seeds, any nutmeats and some commercial mixes.

Don't forget the trunk-feeding birds, such as woodpeckers and bashful chickadees who shun feeders too close to the house. Suet cakes (available at meat counters) mixed with peanut butter are a great treat. Plain suet, too, will be devoured. Put the suet in a mesh onion bag and tack it to a tree.

In placing feeders, remember that predators are always ready to pounce on songbirds. Put the feeder as close as possible to tall shrubs or dense foliage. It will provide a ready escape route should a housecat or hawk be nearby.

Although some people object to squirrels raiding a bird feeder, we don't mind it. If you do, attach a cone-shaped collar of hard, slick plastic or metal around your feeder stand to keep the squirrels from reaching the food -- and hope for good luck. With squirrels, you need it. THE LAST WORD ON BIRDS: Although not cheap at $60, the Audubon Society Encyclopedia of North American Birds by John K. Terres is a monumentally comprehensive reference book. Arranged alphabetically, it provides 800 detailed bird drawings, nearly 900 color photographs and information on habitats, breeding and feeding. Your neighborhood library may carry it.