In celebration of the round-trip ticket to somewhere

WE'RE coming to the season when birds build nests while people, on the other hand, leave home. The Reagans left the White House for China, and the rest of the country is sort of trailing after them - somewhere.

May is the month when Americans get itchy feet, or what S. J. Perelman preferred to call sand in their shoes. Anybody who has any impulse at all to travel is likely to feel that impulse at its most powerful just about now.

Life seems to sing three choruses of ''over the hills and far away,'' followed by a couple of sea chanteys featuring that ''bounding main.'' A little traveling music, maestro.

A salesman who can't sell a mobile home or a 10-speed bicycle in May is in the wrong business. The emptor is definitely not caveating. It's a seller's market in magic carpets and seven-league boots and books advising you how to see Europe on $25 a day.

Travel in the spring is the national rite of regeneration. One comes out of hibernation - moving.

Every May every American becomes a frontiersman and frontierswoman again, with a taste for novel continental crossings that make the covered wagon and pony express look positively prosaic. Any day now the news stories will start to appear, reporting on travelers who negotiate a coast-to-coast odyssey by skateboard. By unicycle. By mule - backward.

Why do people travel, especially in the spring? This can be a nasty question. A lot of the answers will sound thoroughly inadequate. If some of us are being really frank, we may have to admit what Emerson accepted as the true answer: ''I travel in order to be anywhere but here.''

Lucius Beebe, one of the great travelers of this century, as well as one of the great dudes, thought of travel as a form of theater. In the spring one polished one's shoes, slipped a fresh boutonniere in one's lapel, and stepped aboard a perfectly marvelous stage set, like the Twentieth-Century Limited train or one of those gleaming white ocean liners in the slips along the Hudson River that made gala eastern crossings in May.

''It is no accident of circumstances,'' Beebe wrote, ''that the most beautiful devisings and artifacts in the American record have all been associated with motion and movement, the transport of things and people going somewhere else.''

Beebe himself owned a private railroad car, complete with a fireplace and Turkish bath - all done in Venetian decor.

Touring as an elegant star turn may be a thing of the past. But travel for most Americans is still such a pleasure that elaborate rationalizations have had to be invented to hide that fact, like the business trip.

Then there is the ancient argument that travel is educational - broadening. The old Roman, Pliny, shot down this ploy almost 2,000 years ago by noting drily: ''Those works of art or nature which are usually the motives of our travels are often overlooked if they lie within our reach.''

But clever travelers, particularly in May, cannot be denied. They will reply to Pliny's law of tourism as G.K. Chesterton did, by asserting that the very purpose of travel is to learn to ''set foot in one's own country as foreign land.''

Besides, didn't the Founding Fathers recommend the ''pursuit of happiness''? And they did say ''pursuit.''

Like falling in love or planting a rosebush, travel is a springtime fancy finally beyond debate.

Along with the English writer Guy Chapman, all of us wanderers might as well confess at this footloose season, ''I suppose there is something absurd about the intense happiness I get out of the simplest travel'' - and then hurry up and pack our bags.