The case of Northern Virginia's exotic birds was officially closed yesterday after Agriculture Department officials reported that tests on the 2,100 birds they destroyed at a cost of $160,000 showed no evidence of the disease for which they were condemned.

The exotic birds, which were owned or had been sold by Smith's Exotic Aviaries in Alexandria, were killed during the past three weeks because federal officials feared they had been exposed to exotic Newcastle disease, a foreign plague on the nation's billion-dollar poultry industry.

Agriculture officials have maintained that the destroyed birds, from parakeets to rare cockatiels valued at more than $1,000 each, had been exposed to two of Smith's parrots that died from the highly contagious virus.

While officials admitted that none of the birds later killed showed evidence of the disease, they defended their actions as necessary to prevent outbreaks of the disease, such as the 1971 one in California that killed 12 million chickens at a cost of $56 million.

"If we hadn't moved quickly we might have had a repeat of the 1971 epidemic on the East Coast," said Richard Rissler, director of the Agriculture task force that killed 1,600 birds at Smith's warehouse in Alexandria three weeks ago.

Another 500 birds sold by Smith before a quarantine was imposed, were killed by officials who traced them to 120 locations in 13 states and the District of Columbia.

Officials have not been able to trace 200 to 300 other birds sold by Smith during that same period. Dr. Jerry Mason, the veterinarian in charge of all Agriculture task forces, said yesterday that the danger from those birds appeared minimal.

"If the birds had it, they probably died by now . . . and are buried in somebody's back yard," Mason said. "The disease should have run its course by now."

Kevin Smith, the 19-year-old owner of the Alexandria aviary, read the test results as evidence that he had been right in opposing what the federal agency called the "depopulation" of his feathered stock. Smith received $54,000 in compensation from the government for his birds, but he remained unhappy.

"They didn't find any trace of the disease. That just proves how stupid the whole thing was," Smith said.

Agriculture officials said that the yellow-naped Amazon parrots that carried the disease to the Alexandria aviary were smuggled into this country by a California importer who sold them to Smith.

Smith said that if the birds were smuggled, he had no way of knowing that because the importer's name was still on a list of bird importers approved by the Agriculture Department.