What's a songbird doing in the belly of a tiger shark?

The predators are eating land birds affected by offshore oil rigs in the Gulf of Mexico (map), according to new research.

Marcus Drymon, of Dauphin Island Sea Lab, has been studying fish off the Alabama coast since 2006. During a routine sampling in 2009, he pulled a tiger shark onto the deck of his boat to tag and release it.

"He coughed up some feathers," Drymon said.

That in itself wasn't unusual, he said. Tiger sharks in other parts of the world are known to eat marine birds. But once Drymon analyzed the feathers in the lab, he was fairly sure they had come from a terrestrial bird.

So Drymon and his team launched a project to study the sharks' diets. Over two years the team caught 50 tiger sharks—mostly within 5 to 10 miles (8 to 16 kilometers) offshore—and dissected their stomachs.

In about half of the sharks, Drymon found "feathers, or beaks, or bird feet, or some kind of bird part."

All the parts were later found to originate from land birds such as woodpeckers, tanagers, and meadowlarks. (See songbird pictures.)

Sharks Seeking Out Birds?

Oil-rig lights often disorient migrating birds, making them crash into the rigs or fall into the water from exhaustion, said Christine Sheppard, bird-collisions campaign manager for the American Bird Conservancy.

More birds are killed each year by colliding with rigs than were killed by the 2010 Gulf oil spill, for example.

It's possible, she said, that "the sharks may actually be learning there are places where there are birds available to them." (Also see "Pictures: Sharks Taught to Hunt Alien Lionfish.")

Drymon, whose results will be published in a future study, said more research on bird-eating sharks is needed.