STEVE INSKEEP, host:

And our next story takes us a little north of L.A., to California's San Francisco Bay, where a massive restoration project has a tiny problem. The bay has been a site for industrial salt harvesting for more than a century. Thousands of acres of ponds used for that work are now being restored for shore birds and wildlife. But the project has created an opportunity for the aggressive California gull. Lauren Summer reports from member station KQED.

(Soundbite of music)

Unidentified People: (Singing) Let's go Giants. Let's go Giants.

(Soundbite of clapping)

LAUREN SOMMER: It's the bottom of the 9th inning. Giants, three. Padres, five.

(Soundbite of batter hitting baseball)

(Soundbite of cheering, applause)

SOMMER: As the crowd cheers the late-inning comeback, from across the water, hundreds of gulls appear. Like clockwork, they show up just before the game ends.

Mr. MIKE KRUKOW (Broadcaster, New York Giants): Don't ask me how. They just know.

SOMMER: Mike Krukow is a broadcaster with the Giants.

Mr. KRUKOW: They come in, and it's always with two outs to go in the 9th inning, and there they are.

SOMMER: The attraction, of course, is the half-eaten hot dogs and popcorn left in the stands. Gull numbers have grown so high that the ballpark is considering bringing in a falcon to scare them away. But that's not possible everywhere.

(Soundbite of chirping)

SOMMER: At the southern end of San Francisco Bay, a crew from the U.S. Geological Survey is working on a small island in the middle of a former salt pond. It's home to a colony of Forster's terns.

(Soundbite of chirping)

Dr. GARTH HERRING (Scientist): Hey, Nikki what was the band number on that chick?

SOMMER: Garth Herring and a team of scientists are fitting the small tern chicks with radio transmitters.

(Soundbite of radio static, beeping)

Dr. HERRING: When that transmitter is attached to a live chick, the transmitter beeps at a very specific rate.

SOMMER: If the chick dies, the beep slows down. But you might wonder: Why do they need to know if a chick is dead?

Dr. HERRING: Just to the north of us, about roughly a mile, there's one of the largest California gull colonies. They'll come in. They'll grab the chick.

SOMMER: The gulls then feed them to their young. After it's digested, Herring and his team go looking for the still-beeping transmitter.

Dr. HERRING: It's pretty common that we find, you know, just a small pile of bones and the radio transmitter.

SOMMER: Last year, gulls ate 40 percent of the tagged tern chicks.

Dr. CHERYL STRONG (Biologist, U.S. Fish and Wildlife Service): They're the big bully.

SOMMER: Cheryl Strong is a biologist with the U.S. Fish and Wildlife Service. She says California gulls first showed up in the Bay Area in the 1980s. Since then, the population has exploded.

Dr. STRONG: California gulls are one of the earliest nesting species, and they're also probably the most aggressive.

SOMMER: The restoration project has spent millions on pond habitat for shorebirds like Forster's terns and the threatened Western snowy plover. But it's a conservation Catch-22. As habitat is restored for shorebirds, it also creates more habitat for gulls.

Dr. STRONG: With 40,000 gulls, there's not a lot of room for a lot of other birds.

SOMMER: Strong says the Fish and Wildlife Service is studying ways to manage the gull problem. One option is killing the birds. But gulls can live up to 25 years. And with an endless food source at nearby landfills and baseball parks, she says there's only so much they could do.

Dr. STRONG: If you're talking about removing birds lethally, it's just not feasible.

(Soundbite of loud whistle, bird squawking)

SOMMER: Caitlin Robinson-Nilsen is with the San Francisco Bay Bird Observatory. Her job is to haze the gulls. She walks across a levee, blowing a whistle and waving her arms.

Ms. CAITLIN ROBINSON-NILSEN (San Francisco Bird Observatory): So up ahead, there are some gulls doing some courtship behaviors on the levee, which is definitely a bad sign. This is one of the areas where we definitely don't want them to nest.

SOMMER: The idea is to keep the gulls away from sensitive shorebirds. And they aren't happy about it.

Ms. NILSEN: They'll dive bomb you and hit you in your head. They're very good a pooping on you. They have pretty good aim that way.

SOMMER: Robinson-Nilsen says they're hazing gulls twice a day during nesting season at selected sites near restoration areas. So far, it's working. There are fewer nests there. But with millions of tax dollars being spent on restoring habitat, biologists expect they'll be doing a lot more gull management in the years ahead.

For NPR News, I'm Lauren Sommer, in San Francisco.

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