A small white gull with an ordinary name had bird watchers flocking to the Salton Sea for what they call a "mega-rarity."

The Salton Sea, a 35-mile-long lake stretching across the Imperial and Riverside county line, is a popular stop for birds heading south, and Guy McCaskie, co-author of "Birds of Salton Sea," believed he spotted a Ross' gull there a week ago.

The appearance of the arctic bird nearly 100 miles east of San Diego would be the first reported in California and would place it hundreds of miles farther south than it had ever been seen.

The gull, which normally breeds in Siberia or Greenland, rarely appears south of Alaska, and is only spotted in even the northern part of the lower 48 states every few years.

"It was a really fancy bird," McCaskie said.

McCaskie had seen the bird several times on trips to Canada and Alaska and had seen hundreds of pictures, so despite his amazement he had little doubt it was a Ross' gull.

Still, he wanted witnesses.

McCaskie called bird-enthusiast friends who lived nearby, and they quickly appeared and took pictures. He reported his sighting on the Internet news group Calbirds the night of Nov. 17.

By Sunday morning, bird watchers from around the state were racing to the Salton Sea.

"We thought it was worth a 10-hour drive, even knowing we might not see the bird," San Francisco ornithologist Scott Terrill said. "It's a really good bird."

Terrill and his wife arrived to find about 40 people looking for the gull. That number swelled to about 150 as the day went on. Virtually all reported seeing the bird.

Those who arrived later than dawn Monday were too late. The gull flew off around 6:30 a.m.