Tourists on board an 80-foot boat watched an unexpected avian rescue when a baby Caspian tern was found drowning in a harbor.

The 20 passengers cheered when lifeguards, who had responded to an emergency call, scooped up the bird out of the water near a steel barge anchored about a half-mile offshore. The bird was taken to a local bird center.

“This poor little guy is shivering like a marathon swimmer,” said Susan Kaveggia, a biologist with the International Bird Rescue Research Center in San Pedro. “It’s about 2 to 3 weeks old, judging from the length of the feathers on its neck. The fact that it’s breathing through its mouth and has its eyes closed indicates it’s very stressed.”

The bird was likely part of an artificial nesting site for about 350 Caspian terns on the barge that once was a former icebreaker. The slim, gull-like seabirds are protected by the federal Migratory Bird Treaty Act.

The barge’s owner, Sause Bros. of Coos Bay, Ore., has agreed not to move it until all of the birds have matured and migrated elsewhere for the winter. Some worry, however, the birds may be injured or killed in the nation’s busiest port complex.

“The fact that these birds have settled on this barge underlines a critical problem: We need more natural habitat for them,” Kaveggia said.

A year ago, more than 400 Caspian and elegant terns fell off two privately owned barges anchored not far from the Arctic Challenger.