The only California condor egg known to have been laid this season has been destroyed, a small but significant tragedy in the campaign to save the condor.

As dismayed biologists watched from a blind a half-mile away, the 2-week-old egg fell off its cliffside perch during a violent squabble between the two would-be condor parents over who could sit on the egg, condor project co-director John Ogden said today.

The loss of this egg was particularly grievous because the Feb. 14 laying was the first time such an event had been witnessed by scientists studying the condor, project spokesman Inez Connor said. The biologists were stunned to see the mother lay the egg standing up, letting the egg drop nearly a foot to the hard ground.

With 25 to 30 California condors left in the world, the destruction of the egg was a significant blow, but Ogden said he hopes the condor parents, who have since made up, will produce another egg this season.

"There is circumstantial evidence that condors can lay again if they lose an egg this early in incubation," Ogden said in a telephone interview from his headquarters in Ventura, Calif. Normal incubation for a condor egg is two months.

The two huge birds, who lost their egg from its perch in the Angeles National Forest last week, have mated again since the incident, he said.

Biologists of the joint U.S. Fish and Wildlife Service-National Audubon Society project, who have been watching the birds for weeks, can only speculate what caused the tragic parental row, Ogden said. Some biologists have suggested that unusual stress brought on by such problems as malnutrition can cause aberrant behavior in condors.

The two birds began to quarrel over incubation rights after the female returned from several days of foraging.

"One of them would sit down on the egg and the other would come in and try to push the first one off," Ogden said. "They would jab each other in the face . . . , although there was no squawking because these birds rarely make a sound."

The egg rolled to the edge of the cliff and the birds unsuccessfully tried to roll it back. Another squabble ensued, and finally the four-inch egg was knocked out of the cliffside cave and over the cliff, where it smashed on a ledge below and was eaten by ravens.

The 5-year-old, $1 million project to save the bird from extinction suffered a similar setback in 1980, when a condor chick died while being handled by a project member. The ensuing controversy produced a plan for trapping of 2- to 5-year-old condors, who do not reach adulthood until age 6, to attempt breeding in captivity or to tag them with radio monitors so they can be tracked.