American folk art, as represented by hand-carved duck decoys, is depicted on four new 22-cent commemoratives. The stamps show a uniquely American art form, originated by the Indians centuries ago, adapted and improved by the settlers and still practiced today.

Decoys are imitation wildfowl made to deceive live birds and lure them close enough to be killed. The Indians made theirs of reeds and feathers and stuffed dead birds. Finely preserved Indian decoys, going back possibly 2,000 years, have been found in Nevada. The white man, following the impulse of his race for more permanence, made his of wood. Decoy-making reached its peak in the latter half of the 19th and the early 20th centuries, despite competition from factory-made decoys. Today, the carved decoys are the indispensable companions of those who enjoy the sport of wildfowl hunting.

The new issue, which appears as a block of four dissimilar stamps joined together, depicts two from the closing years of the last century, when decoys were still being used in the thousands in the commercial slaughter of birds to provide meat for American tables, and two from the early 20th century, after the slaughter was banned and only a few master carvers continued to follow their art.

There were once enormous flocks annually migrating between their breeding grounds in Canada and their wintering areas in the South. The birds flew along four major "sky highways," along the Atlantic and Pacific coasts, the Mississippi Valley and the Rocky Mountains. Hunting was most intense along the Atlantic coast, the area of the decoys on the stamps and the one where decoy carving developed into a true art form. No other single area produced so many decoys, by so many outstanding carvers, over such a long period of time.

The decoy stamps are the fifth issue in the continuing Folk Art Series that began in 1977 with a block of four depicting pottery of the Pueblo Indians. Successive annual blocks were devoted to basket-design American quilts, household articles of Pennsylvania Toleware and Pacific Northwest Indian masks in 1980.

The designs of the four new stamps are based on actual decoys and include the most commonly sought and plentiful species, such as the mallard and the canvasback, which, at 65 miles an hour, is supposed to be the fastest game bird that flies.

The opening stamp of the block at the upper left pictures a broadbill carved in 1890 by Benjamin Holmes, one of three famous carvers from Stratford, Conn. His decoys were exhibited at the Centennial Exhibition in Philadelphia in 1876. For design purposes, the stamps of the block show the decoys all sitting primly with heads straight forward, the most common position. But more imaginative carvers, such as those in the "Stratford School," often depicted birds sleeping or preening.

A mallard decoy carved by Percy Grant of Osbornsville, N.J., in 1900 appears on the upper right commemorative. New Jersey decoys tended to be hollow and lightweight to keep from overloading the shallow draft boats used by hunters. On the lower left stamp is a canvasback, carved in 1929 by Bob McGraw of Havre de Grace, Md.

Concluding the block at the lower right is a decoy of a redhead duck carved in 1925 by Keyes Chadwick of Martha's Vineyard, Mass., overshadowed in his state only by Elmer Crowell, of Cape Cod, who is regarded by many as the greatest of all decoy makers.

The mallard and the canvasback decoys are part of the collection of the Shelburne Museum in Shelburne, Vt., site of the first day issue. The museum's 1,300 working decoys include many of the finest from the four main hunting regions. It also has an array of wildfowl hunting paraphernalia.

The stamps, designed by Steven Dohanos of Westport, Conn., have been produced by gravure in five colors: black line, yellow, magenta, cyan and black tone. A five-digit plate number preceded by the letter "A" appears on the selvage of each post office pane of 50 stamps along with the customary marginal inscriptions.

The letter "A" precedes all plate numbers of stamps produced by private companies under contract with the Postal Service. All have been produced in a joint venture by the American Bank Note Co., which does all the processing, and J.W. Fergusson & Sons, which does printing.

Collectors of first-day-of-issue cancellations have alternative ways of ordering. Orders must be postmarked no later than April 21.

Those acquiring and affixing stamps on envelopes, which must bear return addresses, should send their covers to Customer-Affixed Envelopes, Postmaster, Shelburne, Vt. 05482-9991.

Those preferring full processing by the Postal Service should send their first-day covers, which must be addressed, to Duck Decoys Stamps, Postmaster, Shelburne, Vt. 05482-9992. The cost is 22 cents per stamp affixed, 88 cents for the block. Those wishing individual stamps affixed must specify the stamp desired. Personal checks are accepted, cash is not welcome, postage stamps in payment will be rejected.