Scientists Fail to Identify Killer of 150,000 Birds

Scientists have been unable to pinpoint why 150,000 eared grebes died at the Salton Sea during 1991-92--one of the nation’s largest die-offs of birds--although researchers have ruled out pesticide poisoning and the toxic elements selenium and mercury as direct factors, authorities reported Wednesday.

“It’s still a mystery,” William R. Radke, wildlife biologist with the U.S. Fish and Wildlife Service, said after a meeting of federal, state, local and private experts.

In fact, scientists still do not know if the grebes--a migratory species that ranges from Alaska to southern Mexico--contracted the killer at the Salton Sea or elsewhere on their annual peregrinations.

Moreover, Radke and others said the precise cause may never be isolated. Studies are continuing, focusing on possible diseases, biological toxins and contaminants.

However, authorities expressed confidence that the North American eared grebe population--numbering 2 million, many of which are breeding in freshwater lakes from California to Alaska to Montana--would survive.

“There have been die-offs before, and there’s still lots of grebes,” noted Dan Audet, environmental contaminant specialist with the Fish and Wildlife Service.

While eared grebes have perished en masse before, the deaths of about 150,000 birds is the largest grebe die-off ever recorded, authorities said. The birds perished between December, 1991, and April, 1992, at the sprawling saline sea, 130 miles east of San Diego.

Experts have expressed alarm about the grebe deaths, wondering whether the die-offs could be a harbinger for other species at the lake.

The Salton Sea, a low-desert oasis, is a crucial habitat for 2 million birds annually, including endangered species such as the peregrine falcon, bald eagle, brown pelican and Yuma clapper rail.

The sea has long been considered on the verge of ecological collapse, largely because it draws pesticides, fertilizers and mineral-laden agricultural runoff from a network of farm drainage canals and takes in residential and industrial sewage from two rivers that enter it from Mexico.

Significantly, though, scientists say it is unlikely that contaminants alone killed the birds. While toxic elements such as selenium and mercury were found in elevated levels in the tissue of dead birds, authorities said, the concentrations were not fatal.

Selenium, which tends to concentrate in some agricultural drainage, was blamed for thousands of bird deaths and deformities a decade ago at Kesterson National Wildlife Refuge in the San Joaquin Valley.

Toxic substances may have played an indirect role in the die-off, Audet said, possibly weakening the birds’ immune systems and making them more susceptible to other ailments.

Also eliminated as potential causes of the die-off were avian botulism, a common killer, viral infections and salt toxicity in Salton, which is saltier than the ocean, scientists said.

Avian cholera, another common killer carried by a bacteria, is still under suspicion, authorities say, as are biological toxins that may have been produced by algae or plankton and consumed by eared grebes.

The grebe die-off is believed to be the largest incident of avian mortality in the United States since the Exxon Valdez oil spill in Alaska killed at least 375,000 birds, mostly sea species, three years ago. In 1969, an outbreak of avian botulism killed an estimated 140,000 birds, mostly waterfowl and shorebirds, in California’s Central Valley, authorities say.