Over 300 dead birds are likely Gulf spill victims

* Mortality toll outnumbers 31 surviving oiled birds

* Brown pelicans, northern gannets hardest hit so far

By Steve Gorman

LOS ANGELES, May 24 (Reuters) - More than 300 sea birds, the bulk of them brown pelicans and northern gannets, have been found dead along the U.S. Gulf Coast during the first five weeks of BP's huge oil spill off Louisiana, wildlife officials reported on Monday.

The 316 birds found dead along the coasts of Louisiana, Mississippi, Alabama and Florida -- plus 10 others that died or were euthanized at wildlife rehabilitation centers after they were captured alive, far outnumber the 31 surviving birds found oiled to date.

The raw tally of birds listed by the U.S. Fish and Wildlife Service as dead on arrival at wildlife collection facilities include specimens obviously tainted with oil and some with no visible signs of oil contamination.

But all are being counted as potential casualties of the oil gushing since April 20 from a ruptured wellhead on the floor of the Gulf because of their proximity in time and space to the spill, said Jay Holcomb, who directs a rescue center for birds in Fort Jackson, Louisiana.

The specimens eventually will be analyzed to determine more conclusively if the birds were contaminated with oil from the BP spill, he said.

Holcomb, director of the California-based International Bird Rescue Research Center, said mortality for sea birds, many of them in the midst of their breeding season, is expected to climb sharply, especially if hurricanes move into the region and sweep more oil ashore.

"The potential for this being catastrophic is right there because there's a massive amount of oil in the water, and it's still pouring out, and there's a lot of nesting birds and a lot of birds using the coast," he told Reuters. "If the tropical storms take that oil and move it, that's when you're going to see the real impact, I think."

DIVING BIRDS HARDEST HIT SO FAR

The birds known to be hardest hit by oil in the Gulf so far are those that feed by diving into the water for fish, including the brown pelican, removed last year from the endangered species list, and the northern gannet, Holcomb said.

But shorebirds, wading birds and songbirds will increasingly be put in harm's way as more oil washes onto beaches and into marshlands.

Oil impairs the insulating properties of birds' feathers, exposing them to cold and making it difficult for them to float, swim and fly. Chemicals in the petroleum also can burn their skin and irritate their eyes. They also end up ingesting the oil when they preen, damaging their digestive tracts.

Other wildlife at immediate risk in the Gulf are sea turtles, dolphins and whales.

More than 150 sea turtles were reported washed up dead or dying at last count a week ago and were being examined as possible oil spill victims, though no outward signs of oil contamination were readily apparent, wildlife officials said.

Still, that number is considered higher than normal for the Gulf region at this time of year. Results from the first batch of about 40 turtle necropsies -- the animal equivalent of autopsies -- were expected soon.

In addition, at least a dozen dead dolphins have been reported as stranded in the spill zone.

(Editing by Pascal Fletcher and Sandra Maler)