WATCHING AS WORKMEN PUT the finishing touches on his giant man-made grotto, J. David Bamberger has an admission to make. "I am," he concedes, "kind of an oddball."

Kind of? Does the phrase "bats in the belfry" ring any bells? Of course, Bamberger doesn't have a belfry. Nor, for now, does he have any bats.

But he does have a nearly finished bat cave, 20 feet high and 40 feet wide, made of 200 cubic yards of Gunite, the stuff from which swimming pools are made, wrapped around 20 tons of steel. And he has a theory, or at least a hope: If he builds it, they will come.

Bamberger, 69, who is building the cave on his 5,500-acre ranch near Austin, Texas, isn't just some bat guano-be. He's a trustee of the Austin-based Bat Conservation International, a nonprofit group dedicated to preserving bats and their habitat.

Bamberger, a divorced father of three grown children, hopes his $250,000 bat condo, which will be the largest man-made bat cave in the world, will accommodate up to a million of the leathery-winged creatures, whose natural habitat has been reduced by development. "I've dedicated my ranch and my life to conservation," he says. "This is an example of what man can do to mitigate the damage we've done to nature."

A multimillionaire who made his money when the Church's Fried Chicken chain, of which he was part-owner, was sold in 1989, Bamberger began building his bat cave in March. But bad weather delayed construction, so he missed a chance to attract bats during late summer, when they swarm in Central Texas like flies. By Halloween, his prospective tenants will all have migrated to winter quarters in Mexico, not to return until April. "I may have to sit here and be a laughingstock, waiting for bats to find me," he says with a laugh. And if they don't? "It'll make one hell of a wine cellar."