ROBERT SIEGEL, HOST:
Across the eastern United States, hundreds of thousands of houses stand empty. Their former residents have fled because they don't like winter. And it's up to an odd group of landlords to watch over these abandoned places. Move over, Michael Keaton. We're going to meet a different kind of bird man. He invited NPR's Adam Cole to see these special dwellings.
ADAM COLE, BYLINE: If you drive around New Zion, South Carolina, you can't miss them.
This is a nice place.
They typically stand in rows, raised on metal poles and painted white. But they're oddly pear-shaped and only eight inches across.
Here we are, another colony.
This is a cluster of hollowed-out gourds. It's an apartment complex for purple martins - native North American swallows. They're not purple, exactly - more like a deep metallic blue. At the moment, though, I can't see a single bird. They all flew off to South America months ago. But come spring the martins will be back. And these gourds will be all a-twitter because here's the weird thing about purple martins - on the East Coast, they nest exclusively in human built housing.
Sure, they used to breed in tree holes and little nooks in cliffs, but then thousands of years ago Native Americans started putting up hollow gourds for the martins. No one really knows why. Maybe the birds ate pesky bugs or maybe they made good alarm clocks, but, anyway, the birds left the natural world behind.
BUBBA JOHNSON: Well, they are probably the happiest birds that you'll ever come in contact with.
COLE: Bubba Johnson is one of the thousands of devoted human landlords who are still carrying on that gourd-house tradition.
JOHNSON: You can say I'm the bird man. They call me the bird man, and everywhere you see a purple martin pole, I probably had something to do with it.
COLE: Johnson is a farmer here in New Zion. He mostly grows cucumbers for pickling, but he also grows gourds for the martins.
JOHNSON: I'm at real (unintelligible) love with them if you want to just tell it like it is.
COLE: And lucky for them lots of people feel the same way. If they didn't martins would be forced to fend for themselves and nature might not be the most hospitable place for them these days. Why? Well, it may be hard to believe, but one of the major people to blame for this is William Shakespeare. Stay with me - see Shakespeare mentions a lot of birds in his plays.
(SOUNDBITE OF ARCHIVED RECORDING)
UNIDENTIFIED WOMAN: It was the nightingale and not the lark.
UNIDENTIFIED MAN #1: I know a hawk from a handsaw.
UNIDENTIFIED MAN #2: Nay, I'll have a starling shall be taught to speak.
COLE: And it's that last one - the starling - that turned into a real problem for purple martins because centuries later in 1890 a New York drug manufacturer had a whimsical idea. Wouldn't it be cool, he thought, if we had every bird mentioned by Shakespeare right here in New York City? So he got a hundred European starlings and released them into Central Park.
Over the next century, those birds multiplied into an army that reached from coast to coast. And now they've invaded the natural nest sites where martins used to breed. Even worse they break into martins' human-built houses. That means Bubba Johnson can't just be a landlord. He has to be a security guard.
JOHNSON: We've got to install a trap that'll trap the starlings.
COLE: And barricades to keep away raccoons and cages to keep away owls and electrified poles to keep away snakes. In fact the purple martins have a whole nationwide network of safe houses. And it seems to be doing its job.
Some songbird species are declining, but purple martin numbers have been steady for decades and you can actually see this success in a really dramatic way. Every year in late summer, before they head south, martins gather in enormous roosts, hundreds of thousands of birds fill the sky in swirling, shifting curtains.
JOHNSON: It is something to see I'm telling you. A person will not believe it until they see.
COLE: I saw it, and he's right. It is incredible. And you can see it too on our science YouTube channel Skunk Bear. By now, all the birds in that video are basking in the Brazilian sun. And back here in South Carolina, landlords like Bubba Johnson are working away repairing old birdhouses, building new ones and waiting for their tenants to return. Adam Cole, NPR News.