Cause of Bird Deaths at Salton Sea Eludes Experts : Wildlife: Some 100,000 eared grebes have died. If cause is traced to a contaminant, other species may be in peril.

Beginning shortly after dawn, workers navigate air boats along the frothy shoreline of California’s largest lake, pausing periodically to scoop up clusters of desiccated corpses and put them in plastic bags.

“They’re dying faster than we can pick ‘em up,” said William R. Radke, wildlife biologist here, as he gestured toward a muddy stretch of waterfront littered with dead birds.

Since December, federal officials say, some 100,000 eared grebes--a shy, migratory diving bird with a pointed beak and ruby eyes--have perished at their winter habitat at Salton Sea, the huge expanse of saline water about 150 miles southeast of Los Angeles.

Many other grebes are ill, apparently seriously dehydrated and gathering pathetically in soiled canals of agricultural runoff to preen and gulp fresh water, however tainted it may be.

The magnitude of the die-off makes it one of the largest to affect a species nationwide in recent years, U.S. officials say.

Glenn Olson, western regional vice president of the National Audubon Society, said: “There probably isn’t a body of water more important in California for water birds and shore birds than the Salton Sea.”

Yet, fully three months after the corpses began washing ashore, the cause of death remains unknown, as researchers continue to examine tissue and other evidence to find out what went wrong in this great inland sea by the agriculture-rich Imperial and Coachella valleys.

“It’s definitely a mystery,” said Steven L. Goodbred, a biologist with the U.S. Fish and Wildlife Service’s contaminants branch in Laguna Nigel.

While the culprit could be a bacteria, virus or parasite peculiar to the grebes, researchers have yet to pinpoint any pathogen, and scientists are looking closely at environmental contaminants--an especially chilling prospect here.

The Salton Sea, up to 40 feet deep and featuring 115 miles of shoreline, has for decades been dependent on the same polluted sources that may be killing it.

Lying in a natural depression below sea level, the vast body of water draws pesticides, fertilizers and mineral-laden agricultural runoff from a wide network of farm drainage canals, some as wide as rivers. Two waterways that originate in Mexico, the New and Alamo rivers, deposit a soiled stew of heavy metals, solvents and other pollutants into the sea.

Long thought to be on the verge of ecological collapse, Salton Sea remains a crucial habitat for 2 million birds annually, including such endangered species as the peregrine falcon, bald eagle, brown pelican and Yuma clapper rail. Exotic birds such as the wood stork, booby and Laysan albatross also frequent its shores. In all, visitors have cited 377 species at the wildlife refuge, making it one of the nation’s most varied bird habitats.

The sea is also a major sport fishery, stocked with orange-mouth corvina, sargo and Gulf croaker, all of which were transplanted from the Gulf of California and thrive in the saltier-than-ocean waters.

Biologists fear that if the ailment killing the grebes is traced to a pesticide or other contaminant, other species could be threatened at a time when such wetland habitats are growing increasingly scarce. They note that California’s seemingly insatiable quest for fresh water has depleted two traditional migratory bird habitats in the eastern Sierra, Owens Lake and Mono Lake.

“This could be really serious in the sense that it could be a harbinger of things we may start seeing more often in the Salton Sea,” said Olson of the Audubon Society.

Some cormorants, herons and egrets have ceased nesting here, raising considerable alarm among biologists that contamination may have interfered with breeding.

And, if the mystery killer does turn out to be a toxin, or some avian bacteria, the prospect of other species taking ill becomes a concern. The grebe, biologists say, may be more sensitive, and therefore may be the first to go.

“The grebe may be like the canary in the coal mine: the sentinel that gives us a warning,” said Goodbred.

However, scientists caution that it is too early to jump to conclusions about contaminants. The grebes migrate as far north as Alaska and as far south as the tip of Mexico and may have contracted their malady elsewhere.

At least one prominent ornithologist, Joseph R. Jehl, director of research at the Hubbs-Sea World Research Institute in San Diego, speculates that the die-off may be part of a natural cycle for the species. About a century ago, Jehl said, naturalists recorded a much smaller loss of about 35,000 grebes at then-pristine Owens Lake.

“This is not a big threat to the species, but it is something to be concerned about,” Jehl said, noting that as many as 1 million grebes gather annually at Salton Sea.

Paradoxically, the Salton Sea would soon dry up without the waste water that is also its nemesis. Almost six feet of water vanishes each year through evaporation, creating the need for constant replenishment. Rain seldom falls in this desert region, and freshwater streams are scarce.

In fact, naturalists worry that efforts to conserve water on surrounding farmland could undermine the sea by decreasing flows and heightening salinity and toxic concentrations. Environmentalists and engineers have put forth grandiose suggestions to salvage the sea, involving building elaborate systems of evaporation ponds, dikes and canals. All seem too expensive to pursue.

The desert sea’s predicament can be traced to its unusual origin. Situated in an area known as the Salton Trough, the sea lies in a basin that historically has served as drainage for the Colorado River. A vast prehistoric body of water, Lake Cahuilla, existed here, drying up 10,000 years ago.

The Salton Sea is a man-made lake that was formed by accident beginning in October, 1905. It was then that a temporary diversion of the Colorado River for agricultural use went awry, sending the entire discharge coursing into the Salton Trough for 16 months. Since then, agricultural runoff has largely kept the lake viable.

Opportunistic birds, including eared grebes, used the sea as a winter habitat and temporary stopover zone. The grebes’ principal diet here, pile worms and their larvae, are a thriving ocean transplant, tossed in decades ago to provide fodder for fish.

“This is a completely artificial environment, but it has become a very critical habitat,” said Radke, the refuge wildlife biologist. As he spoke, a column of dozens of American white pelicans flew overhead, just arriving from the Gulf of California.

The 1 million eared grebes that take refuge each year at Salton Sea fly north in February and March to feed at Mono Lake, Utah’s Salt Lake or to breed at freshwater lakes from Oregon to Alaska.

Eared grebes, weighing about two pounds and with a wingspan of slightly less than one foot, are noted for their spectacular courtship behavior. To attract mates, they appear to dance on water. They are also accomplished divers. Eared grebes are so named for their distinctive golden breeding crests, once prized by hat-makers. In the winter, the beasts don a drab, mostly charcoal-colored plumage.

By the end of this week, scientists studying grebe remains expect to have a better idea of whether a toxin has decimated the population. Tests will determine if the birds’ tissue contained hazardous amounts of contaminants--among them residues of pesticides, including DDT, which was banned in the United States two decades ago but remains persistent in the desert sediment, and mercury, a heavy metal that is used in some fungicides and has been found in Salton Sea waterfowl.

Also, scientists are examining the high levels of selenium--a trace element that concentrates in agricultural runoff--in fish and birds. Authorities blamed selenium for abnormalities and deaths among waterfowl and shore birds a decade ago at the Kesterson Wildlife Refuge in the western San Joaquin Valley, spurring a major cleanup effort.

Complicating the search for clues to the bird deaths is a concurrent outbreak of avian cholera--a sometimes common ailment not linked to the human strain. However, the cholera and at least one reported case of avian botulism appear unrelated to the massive grebe die-off, said Linda Glaser, a wildlife disease specialist at the U.S. Fish and Wildlife laboratory in Wisconsin. “The field is still pretty open,” Glaser said of the biological inquiry.

While die-offs involving large numbers of birds occur with some frequency, U.S. Fish and Wildlife spokeswoman Megan Durham said the 100,000 grebes that have perished at the Salton Sea represent one of the largest such incidents in recent years.

Three years ago, Durham said, the Exxon Valdez oil spill in Alaska killed at least 375,000 birds, mostly sea species such as ducks, loons and cormorants.

At Salton Sea, staff members have been called in from other federal refuges to help collect bird carcasses. The workers have gathered about 22,000 grebe corpses to date, said Radke, who noted that many more bodies have decomposed, sunk to the bottom of the lake or been consumed by predators. Remains not kept for tissue samples are incinerated as a precaution against the spread of disease.

Meantime, healthy grebes have begun their flights north to freshwater mating sites or feeding areas. Along the shore, coyotes and gulls poke at the remains of the grebes that did not make it, as the death toll continues to mount.