Not long ago I wrote about how to make a pond for tree frogs so they could breed without the predation and competition of larger frogs.

Since then I've learned a few things about how to make such a pond. I learned much of it from a particularly intelligent green frog.

I had two little plastic ponds, each about a yard across, set flush with the ground in the front yard. A young bullfrog moved into one. Grey tree frogs colonized the other until this particular green frog moved in.

When he showed up, the tree frogs disappeared. So one day I caught the green frog and carried him over to the pond with the bullfrog.

He seemed to sense danger and didn't linger long. Two days later he reappeared in the other pond. He must have hopped 15 yards across the lawn and through the weeds.

This quickly became an experiment. I would put the green frog back in pond No. 1 and he would move back to pond No. 2, which he saw as his pond.

At first the transfer took one or two days. I never saw the frog make this trip. With practice he covered the route faster. Obviously he could find pond No. 2, which he couldn't see.

I started raising pond No. 2 on a support of bricks and stones. My goal in the beginning was to find out how high a pond's edge had to be to admit the tree frogs and exclude green frogs.

Green frogs aren't made for climbing. Each time I would raise the level the tree frogs would move back.

And then the green frog would somehow get back in. Every time the green frog got in and spawned, the tree frogs would give up and leave.

Today the pond is set on pillars of stones at about 20 inches above the ground. And the green frog and his girlfriend are its only residents, along with their current crop of tadpoles.

How do they get in? I wondered.

So one Saturday morning I caught the green frog and carefully put him on the ground where I could watch him. Then I retreated to the deck with a cup of coffee to observe.

After about two minutes, the green frog hopped toward the pond and disappeared into the surrounding thicket. After several minutes' rest, he began to clumsily climb the pile of rocks.

He struggled, fell back and tried again. About two-thirds of the way up, he took refuge in a crevice for about 15 minutes. Then he reemerged to go up over the top and into his pond.

He's still there today, king of his little pond.

Clearly this frog remembers where his pond is and can get in even though he can't see the water from the ground. I also found that the rockpile makes a structure with ideal hiding places.

I think I could exclude green frogs by making a smooth-sided barrier instead of the rocks. But I've decided I like this intelligent frog.

What can we learn from this? One idea is the concept of home range. Once established, most animals have an area they know. They learn its feeding and hiding places. Animals with a known territory live much longer than those traveling through an unfamiliar place.

If you're making a backyard wildlife habitat, try making ponds. Several little ones will increase the diversity of residents.

Elevated ponds will favor certain kinds of frogs. Add structures like rockpiles to provide hiding places. If you make the habitat good enough, your transients will establish themselves and stay.