̽»¨¾«Ñ¡

When it comes to living in the wild, are out of practice. They've grown so used to birdhouses, they no longer know how to search for nests out in nature. It's a serious conundrum—their populations have since 2002.

Purple Martins are among the first birds to return to the States during spring migration. Clumps of martins depart from South America and make their way to the East Coast, where they settle into human-made nest boxes. Usually these homes are made out of hollow gourds, though some are designed as multi-story apartments made of aluminum, wood, or plastic.

For hundreds of years, humans have provided a safe haven for the martins. But they've contributed to the birds' downfall as well. In the 19th century, the and the were brought to the U.S., causing Purple Martin populations to suffer dramatically. The exotics took over martin houses, and in some cases, massacred the nestlings inside. It was a European invasion—one that continues on today.

Still, in the late summer months, Purple Martins are ubiquitous around South Carolina's . Biologists and birders drift over Lake Murray in their boats to attend the thronging of the birds. This year they were disappointed to find that the birds hadn't turned up. It was a mystery that NPR's Adam Cole (the brain behind the science blog ) had to investigate. "I just kept imagining the amazing spectacle," Cole says. "But the birds weren't there. How did half a million birds go missing?"

Was his successful? Follow Cole in this video as he tries to solve the case of the disappearing Purple Martins.