It wasn't a bird . . . a plane, or even a burning helicopter, as some people thought. It was, in fact, a meteor fireball that frightened some citizens of upper Maryland shortly before daybreak yesterday.

Meteor showers had fallen earlier in the morning over a wide area, from Cumberland to Baltimore in Maryland and from Charlottesville to Richmond in central Virginia. But it was in Montgomery County that the sight of something bright falling toward earth drew particular attention.

The Rockville station of the Montgomery County Fire Department got its first call about the phenomenon around 6:30 a.m. Shortly afterward, two firemen on their way to work reported that they'd seen something, too. Lt. Larry Gaddis, in charge of the Rockville station, heard on his radio that Dulles International Airport had confirmed a meteor shower in the vicinity, but other calls described the falling object as a helicopter on fire.

One caller from the Glen Hill area of Potomac gave what sounded like a precise location for the falling thing: a wooded area near Bentridge Avenue, Trail Ridge Drive, and Reach Road. Shortly thereafter, Gaddis found himself tramping about the area in the freezing dawn chill, looking for an unidentified flying object.

"Somebody saw something," Sgt. Robert Harding of the fire department's communication center said later in the confused morning, "but nobody found it."

There was probably nothing to find, according to Dr. Robert Harrington, an astronomer from the U.S. Naval Observatory.

"It was probably a fireball," he said. "If you look up at the sky for five minutes on a clear night, chances are you're going to see a meteor somewhere. When meteors hit the lower levels of the atmosphere and slow down, they ionize the air around them, which starts to burn. That's what you see when you see a fireball."

Fireballs are quite common in the very early morning, between 2 and 4 a.m. They are much less frequent at an hour when more people are around to see them. Yesterday's 6:30 sighting was unusual because of the time, not the event, Harrington stressed.

Another meteor shower like that which preceded the fireball will take place this morning, according to Harrington, probably between 2 a.m. and 4.a.m. About 20 such showers occur a year, he said, when the earth passes through the orbit of a comet.

Montgomery County's Gaddis seemed somewhat disappointed that his early search and a subsequent later one in a four-wheel-drive bush truck had been unfruitful. "We checked up and down along the Pepco power lines," he said. "If there had been a column of smoke or something it would have been easier."

Not necessarily, said Harrington. "Very few meteorites actually hit the ground," he explained. "And when they do most people cannot distinguish them from an ordinary rock."