The birds act disoriented and go into seizures, the same as if they'd been poisoned by domoic acid, a toxin that occurs naturally in red tide algae. But necropsies on the dead birds haven't revealed other characteristics of domoic acid, and there's nothing else apparently wrong with them, Susan Kaveggia, a wildlife biologist at the center, said.
"To me, this bird looks pretty healthy," she said while examining a dead eared grebe, a water bird that mostly eats fish.
Kaveggia took blood and tissue samples from the birds for testing, and the center will be talking with Orange County vector control and healthcare agency officials, as well as state and federal agencies that handle wildlife and the environment. U.S. Fish and Wildlife Service officials also have asked for samples to do toxicology tests.
It could be some sort of bacteria or something in the water poisoning the birds, but McGuire said the problem has affected a wide variety of bird species that feed differently, so it may not be something they're eating.
The affected birds include western grebes, brown pelicans, American avocets and rhino auklets.
According to water-testing data, bacteria levels around the river mouth haven't been higher than normal, except on Feb. 8 and Feb. 10 at Magnolia Street, and there haven't been any beach closures recently, said Monica Mazur, an environmental specialist with the Orange County Health Care Agency.
While the birds' illness could be caused by something in nature, it's unusual.
"In the nine to 10 years that we've been here, we've never had this many birds come in that were seizing when it's not domoic acid poisoning," said Lisa Birkle, the center's assistant wildlife director. "We don't know what these birds have gotten into."
At least one bird was brought to the center by Newport Beach Animal Control, but animal control Officer Jamye Rogers said their logs don't show any more dead birds than they normally see after a rainstorm.