An estimated 10,000 black birds, mostly starlings and grackles, have invaded a four-block wooded area in the Chandon neighborhood of Herndon, Va., leaving a sour smell in the air and droppings all over everything.

The birds arrived in one huge black cloud at about 8 o'clock one evening four weeks ago. They have stayed every night since, leaving in the day to feed on area corn fields, residents of the 400 block of Arkansas Avenue say.

The Fairfax County Health Department is trying to determine whether the birds are a health hazard and has called in a bird expert from Virginia's State Department of Agriculture, the area residents said yesterday.

Roy W. Eidem, field sanitation expert for the country, who neighbors said inspected the area, could not be reached for comment.

"About 8:30 p.m. the sky turns black with birds," said Gary Broersma, a building contractor who lives at 421 Arkansas Ave.

"It's like being in the middle of the bird aviary at the National Zoo," Broersma said.

Two brids got into the Broersma home a couple og weeks ago by flying down a chimney, Broersma's wife Laura said.

John Clayton, director of environmental health for Fairfax County, said he had not heard of the Chandon problem but the normal procedure would be to get a bird expert from the state Department of Agriculture to test the soil for a fungus, sometimes found in bird droppings, that may be dangerous to humans.

The best way to get rid of the birds is to make a lot of noise every 15 minutes throughout the night to keep them awake, Clayton said. Aside from spraying nixious chemicals in the area, which could harm all wildlife, the only other way to get rid of such a flock quickly is to cut down the trees they are in, he said.

But it's best to let the birds leave on their own accord, according to James F. Phillips, senior environmental health sanitarian. "Driving the birds off is not always a good solution because it just shifts the problem to another neighborhood," Phillips said. "It's mainly just a nuisance you have to put up with."

One of the most famous bird invasions occurred in 1974 in Graceham, Md., a community of 400 residents who made money selling "Birdcountry" buttons to tourists. Large flocks of birds have also created problems in the past couple of years in the Sterling and Vienna areas of Virginia.