Who says Florida has no seasons?

Miamians recognize autumn and they don't need a bunch of brittle leaves to tell them.

They wait for an invasion from the north.

"It's like when the swallows return to Capistrano," said Connie Borgschulte, assistant to the Dade County manager.

It's not tourists and ti's not swallows.

It's turkey vultures. You know, buzzards. Those big black birds that hand around waiting for things to die. When they arrive here from their summer roosts in the North they head straight for their favorite winter sunning spot -- the Dade County Court-houes in the heart of downtown Miami. By the hundreds.

According to Dr. Oscar Owre, an ornithologist, Miami has the biggest urban roost of buzzards in the world. He used to believe it was the only one.

He thought so until that claim was in a newspaer. "I received an angry letter from Washington right away, saying there was a roosting population in that city, at the National Zoo," he said.

No matter. Shelia Gaby, who is researching her doctoral thesis on the migratory habits of turkey vultures here, said she regards the Washington population as artificially induced. It's attracted by the free carrion porovided zoo animals.

The Miami population lives off a much more natural phenomenon, she said -- the 58th Street dump.

There are about four nighttime buzzard roosts in Miami, Gaby said, but the roost that occasions the most comment is the daytime one at the courthouse, a 25-story white skyscraper.

The birds that use it create a fantastic spectacle, soaring back and forth to their perches atop ledges on the top few stories.

Tourists stip and crane their necks. Cameras click.

Borgschulte's office used to be on the 22d floor and the birds soared by her window, casting a shadow clear across her office, she said.

There are jokes, of course. "People say the birds picked the courthouse because they always hover around dying institutions," Borgschulte said. "They say the smell of government attracts them..."

Gaby and Owre claim kthat the buzzards' selection of a roost has in the past angered the county commission.

"The commission on several occasions suggested that the vultures be gotten rid of because they don't give the city a good image," Owre said.

He said the Tropical Audubon Society has fiercely opposed any efforts to eliminate the migrant birds.

Borgschulte denies there has been any attempt in recent years to chase the birds.

"Maybe 20 years ago they might have considered it," she said, "but now I think people are more conscious of the need for wildlife to survive."

She said they only impetus for clearing out the flock she knows of came from exterminators looking to make a fast buck.

"We get letters," she said. "They have some crazy ideas. One wanted to build a pyramid over the top nine floors so they wouldn't have any ledge to perch on. Another said he had some pellets that would kill them, but not until eight or 12 hours later, so they'd fall out of they sky somewhere else."

She said all the ideas have been rejected, and that the only problem the birds have created is from feathers and droppings they leave behind. Once they clogged the storm drains. When it rained water backed up and damaged interior walls and ceilings down to the ninth floor.

The commissioners remained unperturbed.

The buzzards "are a beautiful and unique feature of our downtown area," said Borgschulte. "I can sit and watch them soar for hours."

Buzzards are among the great soarers of the avian world.

While birds of prey -- hawks, ospreys and eagles -- mix their soaring with great bursts of flapping speed, buzzards are the mellow, laid-back kings of gliding.

"Just the beauty of their flight is enough to draw attention to them," said Gaby, who spends long hours at the dump tagging buzzards for her migration reaserach.

"They are fantastically efficient gliding animals," she said. "They use thermals far more efficiently than birds of prey."

Not all the turkey vultures are in Florida. Many remain in the Washington area, although Haby thinks these might be migrants from still further north.

The birds glide best on calm, sunny days and are most easily identifiable by their red heads, if you get that close, or by the lateral silver section that runs the length of the wings.

The wingspan is six feet. Mature birds weigh three to five pounds.

Should you become a buzzard watcher, keep an eye out for ones that bear Gaby's three-by six-inch wire ags, with a number and letter on each. Should you see one, write down the number and send data on the sighting to her at 6832 SW 68th St., S. Miami, Fla. 33143

She wants to learn all she can, and she's one of few people researching buzzards. Interest is increasing, she said, because of the plight of the turkey vulture's close kin -- the California condor.

"The condor is endangered," said Gaby. "Perhaps as severely endangered as any species in America."

She wants to make sure the same fate never befalls the scorned but elegant turkey bulture.