COLUMBIA - The early arriving purple martins stack up on the high voltage power lines, tens of thousands of them waiting as if they don't want to rudely arrive at the event too early.

Then the time arrives, marked by the leading edge of the sun sinking below the horizon, and the little swallows with the distinctive forked tails take off to their nightly party. Those first few thousand glide low about a mile across the slightly choppy waters of Lake Monticello and begin circling a small island.

They soon are joined by thousands upon thousands more who had more accurately timed their arrival from as far as hundreds of miles away. Soon, hundreds of thousands of birds dance in unison, adding their own ever-changing cloud to the intricate sunset sky.

The extreme sounds nearly rival the sight - the deafening high-pitched calls along with the whoosh of so many little wings moving the humid night air.

By the time the last of the stragglers arrives 30 minutes later, it is clear that this unnamed island has displaced Bomb Island as the hot summer nightspot in the Midlands for purple martins.

The experts watching the show from a boat debate whether the Monticello flock this summer is larger than the flock that roosted for about a quarter of a century on Lake Murray before mysteriously abandoning Bomb Island this year.

Julie Hovis, an endangered species biologist at Shaw Air Force Base, had been out on Lake Monticello to see the new roost for the first time one night earlier. At that time, she was convinced this roost was larger. One night later, she wonders if it was a matter of perspective. The new roost is smaller than Bomb Island, so maybe the swarm of birds just appears to be larger here.

Then again, maybe it really is.

"They're thick over the island," Hovis says. "Then you look back and they're that thick again, and again, and again."

When the 30-minute show is over, S.C. Department of Natural Resources wildlife technician Lex Glover weighs in. "I think it's as big as Lake Murray," he says, "but it reaches a point where you just say: It's big."

Roost history

Researchers with expertise in counting large gatherings of birds estimated the Lake Murray roost drew 750,000 to 1 million birds, accompanied by maybe a hundred boats filled with bird lovers during the peak weeks in late July and early August. The new Monticello roost seems to be the temporary summer home to at least that many birds, but one thing is clear: The bird-to-boat ratio is much higher on Lake Monticello.

Earlier in the day, Glover and two other DNR staffers took a trip to Bomb Island to see if they could find hints of why the purple martins decided to change addresses this summer.

The migratory birds spend fall and winter in South America and return to North America each spring to nest in small groups. After their young fledge the nest, they spend the rest of the summer chowing down on insects on trips that cover hundreds of miles during the day. Each night, they return to a large communal roost.

Locals on Lake Murray say the Bomb Island roost began to build sometime in the late 1980s. By 1994, researchers were estimating the roost numbers in the hundreds of thousands. Word spread of the spectacle, which became a summer ritual for local boat owners and lake residents.

But this year only a few hundred purple martins showed up in late June. Bird experts began to notice that the distinctive circle created on weather radar images when the birds leave the roost each morning was showing up on Lake Monticello for the first time.

Hovis took a boat to Bomb Island one night last week to verify the move firsthand.

She counted five purple martins.

A day later, the DNR crew explores the thick underbrush on Bomb Island, which as a designated purple martin sanctuary is off-limits to people this time of year. Working ahead of the others, Glover spooks a great horned owl, the type of predator that might be a suspect in this mystery. But Glover finds none of the whitewash owls leave on trees that are their full-time home. The owl was probably a short-term visitor.

The crew finds the partial skeletal remains of a goat in a small clearing, but it is obviously several years old. (Goats were placed on some Lake Murray islands decades ago to clear underbrush.) Not only is this goat not a suspect in the bird disappearance, but its bones have none of the gnaw marks typically left by rats. That seems to rule out another predator suspect.

Eagle-eyed Amy Tegeler, DNR's bird conservation coordinator, uncovers several small bird skulls. The fact they aren't crunched and in the pillows left by owl pellets seems to support the innocence of the owls. The birds appear to have died of natural causes.

In a clearing at the west end of the island, a fire ring, rake marks in the dirt and a rope swing make it clear people have camped illegally on the island recently. Could they have scared away the birds? Probably not. Purple martins nest near people all the time. In fact they now rely almost exclusively on people to hollow out gourds for their nest cavities, Glover says. Considering the deafening bird calls and quantity of bird feces in a roost area, it's more likely that the birds would shoo campers than that a few campers would shoo the birds.

As the DNR crew left the island, Tegeler says the evidence is inconclusive.

"My hunch is it's just a natural thing," she says. "Roosts move."

Changing patterns

Why the birds went is in doubt but not where they went. The morning radar images indicate some might have gone to Lake Thurmond on the Savannah River, and a roost on Lake Moultrie seems to be growing. And the Lake Monticello roost went from nothing to huge in one year.

Hovis tried to guess which island would be home to the roost on her first visit to Monticello in mid-August. Looking out from the public boat landing along S.C. 215, she could see two islands straight ahead. One is large and covered with tall trees, all about the same height. The other island is much smaller with a variety of types and heights of trees.

"I saw that island and I said, 'That looks perfect, but it's way too small,"' Hovis says.

Noticing the birds staging on the wires leading from the V.C. Summer Nuclear Generating Station, Hovis guided the boat to that southern edge of the lake. There the crew waited for the birds to lead them to the roost. To her surprise, as the sun neared its 8:24 p.m. set, the crew was led back to the small island.

A night later, she is still amazed to watch as the birds end their dance in the sky and settle down on branches for the night. They are packed so dense, they bend the limbs of the large loblolly pine in the center of the island.

"Did you see that?" Glover says as the birds alight almost all as one, "it's like all of the vegetation on the island suddenly moved."

The tree limbs on the larger island 200 hundred yards away sit empty.

The islands on Monticello don't have official names on maps. The crew on the boat decides the roost must have a name. Martin Island is too simple. They decide it should be more distinctive. In honor of the purple martins' scientific name, Progne subis, they decide Progne Island would be appropriate.

Of course, giving the island that name might be premature. After all, roosts move, as this summer has made abundantly clear. They could move back to Lake Murray next year. Or they could hopscotch over to Lake Wateree.

"To me, it'll be really interesting to see where they roost next year," Hovis says.