The terns wheeling and screeching overhead don’t seem to know that the island they’ve taken possession of is a kind of imposter.

The platform is a life raft of sorts: a floating island for nesting birds, covered with sand, vegetation and tiny shelters and anchored to the bottom of an Anaheim water-storage basin.

“The babies are darling,” said Orange County Water District biologist Dick Zembal as he paddled around the island this week in a tiny boat. “Wait, that’s not a scientific word. Handsome.”

The 3,500 square-foot island is  Zembal’s invention, and solves a problem that has plagued the agency for the past three years: their Burris water storage basin, a former quarry, became a favorite nesting spot for endangered California least terns and other sensitive bird species.

“They’re colonial nesting shorebirds, 10 miles inland,” said Dan Bott, the water district’s principle environmental planner. “We were all at first taken by surprise that they were there, and they kept recurring year after year.”

But the district must raise and lower the water level there as storage needs dictate. That led to a sticky problem. The district did not want to drown the bird nests, but at times of peak storage, water levels threatened to overtop the islands the birds favored. When the water levels were lowered, a land bridge would allow coyotes and other predators to reach the island.

Zembal said he encountered skepticism in the Orange County birding community when he proposed creating the floating island. But the birds appear to have validated his hunch.

Nature, of course, has its quirks. The island was created with California least terns in mind. But the birds that dominate there are Forster’s terns. Sixty seven chicks have hatched on the island, with 36 eggs unhatched.

Black skimmers, bigger than Forster’s terns, appear to be casing the island for possible nesting as well. But the least terns this year have chosen to nest on the bank of the basin. Their nesting spots are surrounded by fencing so the chicks can stay corraled, reducing the chance they’ll be taken by predators. A water district employee monitors the nesting areas during the day to keep both people and potential predators away.

On Tuesday, Zembal, who has a permit to do so, picked up one of the first least tern chicks to hatch inside one of the enclosures.

Eventually, the sandy bank close to where the terns are nesting will be turned into a 2.4-acre nesting island that will remain clear of the water even when the storage basin is full.

But the $130,000 island was needed, Zembal said, because the work cannot be completed quickly enough to accomodate all the birds that favor the Burris basin this nesting season.

Nesting season ends roughly by Sept. 15; by November, heavy rains become more likely. So grading and reconfiguring the 125-acre basin will take more than one season.

“In the hope to retain the colony of birds here, that’s why we came up with that,” Bott said.

The Forster’s terns took to the air in a storm of apparent protest as Zembal and Bott approached in their inflatable boat, stepped onto the island and added some cuttings from mulefat, a native shrub, to help shelter the birds. They updated their counts of the chicks as well.

The birds’ chicks headed for the water, paddling out some distance while the biologists worked. Others hid beneath tiles and half sections of pipe placed on the island for shelter.

Zembal said he keeps his visits to the island to a minimum, and tries to work quickly to minimize disturbance to the birds.

Unlike California least terns, Forster’s terns are not federally endangered, although they along with black skimmers are considered species of concern by the state of California.

The design of the island is deceptively simple. It is anchored at its four corners with cement-filled trash cans that rest on the bottom; the island can be raised or lowered to match water levels using winches at each corner.

A wire mesh skirt around the edges allows chicks to climb down into the water and back up again.

The island will probably remain in place, rather than being removed, even after the larger island is built.

Zembal said that because of its exposure to predators, the Burris basin is not an ideal nest site.

“This is a stupid place to manage for terns,” he said. “Sooner or later the red-tailed hawks or the kestrels are going to find them.”

When the work was done and the birds settled back down onto the platform, it was time for Zembal to check the least tern colonies, where he found the least tern chick.

“This is not a spot I would choose for them. But it’s not up to me.”

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