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In late July and August, something remarkable happens in the air above Lake Murray in South Carolina. Around sunset, hundreds of thousands of purple martins come streaming toward the center of the lake from every direction, swirling together in a massive flock that darkens the sky. After an hour or so, they settle down on Bomb Island in the center of the lake.

It's called a "roost" — a gathering of purple martins that precedes their epic migration to South America. For the past 25 years, Lake Murray has hosted the largest purple martin roost in the United States. Every year, hundreds of boats full of purple martin admirers crowd the waters around the island. Every year, 500,000 birds put on a breathtaking aerial performance.

But not this year. This year the birds didn't show.

The Spirit of Lake Murray cruise line has already emailed its regular customers warning them of disappointment. A few interested biologists and birders are scrambling to figure out where the birds have gone. The first place they looked: Doppler radar.

Birds leave their roosts simultaneously in the morning and disperse in every direction, to hunt insects all day. They are so numerous that this migration shows up as an expanding ring on Doppler radar.

Based on the radar images, experts think most of the birds are now roosting somewhere on nearby Lake Monticello, with a portion of the birds moving southeast to join a roost on Lake Moultrie. Biologists from the South Carolina Department of Natural Resources say the reason for this dramatic shift is unclear. It could be that the flock has grown too large for the island. Purple martins prefer open spaces, so it could be that the island has become too overgrown with trees.

Whatever the reason, purple martin enthusiasts hope to find the missing birds soon so they can make sure the new site is protected from human interference.

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