One summer day a few years ago I improvised a birdbath out of a large plant pot about a foot tall and a foot wide.

I put a little branch in it so the birds could walk down the branch to reach the water. I often saw the brown thrasher use this watering place to get a drink.

One day, after a rainy night, I looked down into my plant-pot birdbath and saw something new. Frog eggs!

How did they get there?

They were the eggs of the gray tree frog, Hyla chrysoceles. Hyla, like others of his tribe, is a good climber. He can climb a wall or a tree, and even plastic pots.

Obviously some tree frogs had climbed into the pot for a private pool party. Pond privacy for a gray tree frog is important, because they make easy prey for predatory frogs.

The gray tree frog is adept at finding little ponds that aren't being used. And further, it often avoids breeding in little ponds that are well- stocked with other kinds of frogs.

Not only do they avoid predation this way, but their tadpoles can grow up with less competition from other frog tadpoles.

So the habits of Hyla got me to thinking. I could make a special tree-frog pond.

Since then I've tried several designs that attract tree frogs but exclude other kinds of frogs.

All frog ponds should have a tapering shore so the frogs can climb out easily. The best all-purpose edge has a shore set flush with the ground. That way the frogs can easily jump in or climb out.

A variety of frogs may colonize this pond, from tiny peepers to large, cannibalistic species, like bullfrogs and green frogs.

Avoid cliff-like edges that will prevent escape once frogs get in. Ponds with overhanging edges tend to be death traps. Little animals can jump in, but they can't get out.

The key to making a tree-frog pond is to design one that keeps most kinds of frogs out while letting tree frogs in. To do this, make a pond with a vertical edge on the outside.

Here's one way to make a tree-frog pond. Buy one of the little plastic pools that children play in. Instead of submerging it into the ground as you would for most water animals, just put it on top of the ground.

Arrange a bank of rocks or bricks around your little pool. Make it steep. That way, the tree frogs can climb up. But the nonclimbing, clumsier species can't.

Put some branches in the water for extra cover and for the frogs to cling to. Or you can make a little island of wood or stones.

How high should the edge be? I think eight to 10 inches should work. If bullfrogs should find their way in, raise it another six inches.

This should make a pond that will allow your tree frogs to have a home-sweet-home all to themselves.