Early on an April morning, when the sun still hangs low in the Florida sky and the only other early risers are seagulls and herons, a car with out-of-state license plates pulls slowly out of a condominium parking lot. The retired couple inside, maps and travel guides stacked neatly between them, take one last look back at the pool, the clubhouse, the tennis courts, the neatly landscaped grounds - everything that has been their home for the past four months. Then, swinging their car onto a northbound highway, they begin what will be a two- or three-day trip, retracing the route that brought them here in January.
This scene, repeated daily across the Sunbelt, has become a rite of spring. Winter is over, and the snowbirds are going home to tend lawns, put up screens, renew friendships, and wait for what one man calls ''our second summer.'' Having escaped the snow and cold of winter in the Midwest, Northeast, and Canada, they will now escape the bugs and humidity of Florida's summer. ''The best of both worlds'' becomes a cliche quoted again and again by these citizens of two worlds.
Officially, at least, Florida claims to be as pleased with the arrangement as the snowbirds. When the April emigrants heading north return eight months from now to settle into plush oceanfront condominiums, mobile home parks, recreational vehicle campgrounds, and downtown rooming houses, they will again become the darlings of local governments and businesses for the millions of dollars they pour into the local economy. Even the phone company woos them with large newspaper ads touting ''seasonal service,'' which allows customers to pay only a nominal charge during their months away.
A perfect exchange program all around? Not quite. If they bring purchasing power, the snowbirds also strain water resources and sometimes the patience of local residents, many of whom were once snowbirds themselves.
''Many permanent residents are frank to say they like May through November best,'' notes Florence Van Syoc of Godfrey, Ill. ''They don't have traffic, and they don't have to stand in line to go to a restaurant.''
''Come Jan. 1, restaurants change menus and prices go up,'' says Kaye Sommer, an artist who, with her artist husband, divides the year between Tryon, N.C., and Florida.
For the visitors, the ''best of both worlds'' is not always as ideal as it first seems either. There are formidable logistics. Behind the snowbirds' carefree, suntanned appearance lies a carefully orchestrated, often complex system of checklists and countdowns at both locations. Newspapers must be stopped, mail forwarded, magazine addresses changed, the unoccupied home looked after.
Some who own Sunbelt retreats play an expensive game of ''doubles'' - buying two of everything, such as appliances and linens, one for each residence. Other owners and many renters settle for a more economical arrangement: lugging everything from toasters, towels, warm-weather clothes, and spices back and forth.
''To have two homes, you've collected two bags of earth weights,'' says Bryce Van Syoc. ''It would be simpler to live in one place.''
Besides the shuffling of belongings, there is a migration of the heart. As more than a tourist but less than a resident, the snowbird can become lost in a sort of no-man's land.
''You can't get really well acquainted with people,'' says Kaye Sommer.
Florence Van Syoc outlines the pattern: ''The first question people ask is, 'Are you here permanently?' If they have a sense that you're temporary, they're a little less interested in you.
''The second question is, 'Are you renting or do you own?' In other words, 'Are you just here for this year or will we see you again?' ''
''You sometimes end up feeling like a visitor both places,'' says one woman, a veteran snowbird. ''Even your friends back home don't seem quite sure about which address is really your home.''
But as the Van Syocs and many other part-time residents will attest, the advantages can be numerous. Being a snowbird allows people to maintain their family home and their proximity to children and grandchildren. It also permits an exploratory step into the world of Sunbelt retirement.
Ultimately, for many, the time comes to give serious thought to what Mrs. Van Syoc describes as ''the third question local residents ask: 'When are you going to move down here permanently?' ''
''Most of those who make the break are quite happy they have,'' she observes. ''But one woman did say to me, 'Don't be in a hurry . . . don't be in a hurry.' ''
That may be good advice. Recently released census information identifies a growing trend toward countermigration - the movement of elderly Florida retirees back North, usually to be closer to families.
Meantime, the snowbirds stay on the wing - keeping up their membership in two flocks at once.
''You can't sit back and wait,'' Mrs. Van Syoc advises. ''It's a matter of reaching out. You don't have to be aggressive, but just very approachable. I think it's a matter of getting out of yourself and finding some people who have similar interests. It's fascinating the way people open up when you give them a chance.''
For some winter visitors, reaching out takes the form of volunteer work. When the appeal of an extended vacation palls, they offer their services for meals-on-wheels deliveries and school tutoring programs.
Kathleen Ayre, a part-time resident from Hancock, N.H., spends two mornings a week at the Pinellas Association for Retarded Children, working with children between the ages of 1 and 3.
''I went down to PARC last year and asked if they needed a volunteer,'' Miss Ayre says. ''Up until then I was busy seeing friends each year. But I felt I wanted something more to do.''
To be a snowbird is clearly an active state. Completing a round-trip journey requires a lot of old-fashioned American pep, not to mention gumption. But what finally touches the heart of a bystander wishing snowbirds in April a bon voyage is the innocent hope that goes into their act. The native brand of hope - hope as something that moves, the expectation that brought Americans to the New World in the first place - this hope lends a certain gallantry to the trip. It is as essential as the ham sandwich and Thermos jug without which no snowbird would dream of leaving home.