A barge that spilled 168,000 gallons (635,000 liters) of oil Saturday into Galveston Bay is threatening a refuge that's crucial habitat for thousands of birds, experts say.

The spill occurred when the barge collided with a ship in the Houston Ship Channel near Texas City, on the western coast of Galveston Bay.

The area is about a mile (0.62 kilometer) from the Bolivar Peninsula, which is home to the Bolivar Flats Shorebird Sanctuary, a preserved area of marshy mudflats that's home to a variety of geese, ducks, herons, and other waterbirds.

View Images NG STAFF; ANA M. MYERS. SOURCES: HOUSTON ADVANCED RESEARCH CENTER; IUCN; UNEP

The sanctuary has been designated by bird-advocacy organizations as a Globally Significant Important Bird Area and is part of the Western Hemisphere Shorebird Reserve Network.

The government's cleanup efforts began immediately, with 24 response vessels working to skim the oil and to stop the leak from the damaged barge, which was carrying more than 900,000 gallons (3.4 million liters) of oil.

About 69,000 feet (21,000 meters) of oil-absorbing boom have been placed around the site of the spill and along sensitive shorelines, according to the Coast Guard.

At least 50 oiled birds have been discovered so far, though the number will likely be much higher as rescuers expand their search, said Richard Gibbons, conservation director of Houston Audubon.

The animals are taken to a wildlife rehabilitation facility established by Unified Command, a network of agencies overseeing the cleanup.

The Texas General Land Office has deployed a bird rehabilitation trailer in the area for quick response to affected wildlife, according to the Coast Guard.

Compared with the 2010 Gulf oil spill, which oiled hundreds of birds, "this is a tiny amount of oil in comparison—but it is very, very close to a lot of very, very important places" for birds, Gibbons said.

The proximity to the sanctuary, he said, "makes it something we have to be diligent with." (See "Gulf Oil Spill Pictures: Ten Animals at Risk.")

View Images A dead bird covered with oil lies near the Houston Ship Channel on Sunday. PHOTOGRAPH BY MELISSA PHILLIP, HOUSTON CHRONICLE VIA AP

Protecting Bolivar

The Bolivar Peninsula, which lies east of the collision, has not been directly impacted by the oil so far, Gibbons said.

Winds and currents have pushed much of the oil south toward Pelican Island, where the oil is coating rocks along the shoreline.

Shorebirds that sit on rocks, such as gulls; wading birds like the ruddy turnstone; and ducks are being hit hard there, Gibbons said. (See National Geographic's pictures of waterfowl.)

Right now, Houston Audubon's main priority is keeping the oil away from Bolivar.

"We're concerned about all the birds that are oiled, but we have the most control and responsibility for our own property," Gibbons said, referring to the fact that a coalition of bird groups manage the Bolivar Flats sanctuary.

To that end, responders are using hazing cannons—guns that produce loud bangs—on a thin levee of land called the Texas City Dike, which juts into the bay directly west of Bolivar. The hope is that the sounds will keep birds from flying or swimming toward the peninsula.

Ironically, Gibbons said, Bolivar Flats is an ideal location for birds because of the Houston Ship Channel.

The ships' movements break up the bay's natural currents, so there's less disturbance of sand on the beaches, which in turn allows for more crabs, clams, and other tiny creatures—birds' favorite prey—to thrive.

View Images A flock of black skimmers fly over Bolivar Peninsula. PHOTOGRAPH BY AMAR and ISABELLE GUILLEN, ALAMY

Why Oil Is Toxic to Birds

Oil harms birds in two ways: internally and externally.

Oil-soaked birds vigorously preen their feathers to remove the toxic crude, accidentally ingesting it, which can cause internal ailments.

"Try removing crude oil from your arm with your mouth and not getting any of it into your body," J. V. Remsen, Jr., an ornithologist and a professor at Louisiana State University's Department of Biological Sciences in Baton Rouge, said in an earlier interview.

"How much damage has already been done [by the time the bird is rescued]? That's the big unknown in all of this rehabilitation." (Related: "Oil-Coated Gulf Birds Better Off Dead?")

Externally, oil degrades the birds' waterproofing oils, allowing cold water to touch their skin and making the animals vulnerable to hypothermia, especially in cold regions, said Nils Warnock, then of the Oiled Wildlife Care Network in California and now with Audubon Alaska, in an earlier interview.

"It's like surfing in the northern Pacific," he said. "If your wet suit gets a hole, you're going to be cold fast."

It takes weeks or even months for the birds' waterproofing to be naturally replenished. The long recovery time further stresses the animals and reduces space in rehab facilities for incoming patients.

Where Will the Oil Go?

Most of the oil slick seems to be heading southwest down the Texas coast, away from Bolivar. It could go all the way down to Padre Island National Seashore—and that would be good news, Gibbons said.

That's because the longer oil stays in saltwater, the more it's eaten by bacteria. Eventually, the thick, sticky stuff becomes well-formed, easy-to-collect tarballs that are less likely to harm birds.