LOS ANGELES -- California brown pelicans, which were driven to the brink of extinction in the last century, are in trouble again.
An annual survey completed last month found a drastic plunge in the population of breeding pairs, according to a statement released Friday by the University of California, Davis.
The survey in Mexico's Gulf of California -- where about 90 percent of the pelicans typically breed and raise their chicks -- found that areas that typically host hundreds or thousands of nesting pairs held far fewer, and a few places were completely empty, the statement said.
"That's what we call a failure, a bust. The bottom dropped out," said Dan Anderson, a wildlife biologist and UC Davis professor emeritus who conducted the survey along with members of Mexico's National Commission of Natural Protected Areas.
The reason for the decline could range from food supply shifts to changes in water temperature.
Many birds arrived late to the Mexico breeding grounds this spring and "of those who nested, many abandoned their nests when they could not find enough food to sustain their stay," the UC Davis statement said.
The bird's range extends from Mexico to Canada, according to the National Park Service.
Last month, thousands of California brown pelicans moved up the Southern California coast and even as far north as Washington, hunting their main prey of sardines and other fish.
Breeding population crashes of the pelicans often are associated with a warming of the central Pacific Ocean, known as El Nino, but that isn't expected to begin until this summer and the drop also was much steeper.
"During most El Nino events we've seen, numbers of nesting attempts drop by at least half to two-thirds, and production goes down, too," Anderson said, according to the UC Davis statement. "But it drops from thousands to hundreds, not to 10 or less."
The California brown pelican was declared an endangered species in 1970 after its population was pushed to the brink of extinction by the pesticide DDT, which caused the bird's eggshells to become so thin that they broke. After DDT was outlawed, the bird made a recovery and was taken off the list in 2009, when the West Coast population was 150,000.
However, the species has faced new challenges since then because of a decline in sardines.
In 2010, wildlife rescue centers in California were filled with emaciated pelicans. The same year, young pelicans attacked murre nesting colonies in Oregon, shaking the chicks until they regurgitated fish, then eating the fish.
They did it again in 2011 and 2012.
Last fall, scientists said they were concerned that a crash in the West Coast population of sardines might also be starving the brown pelicans. The 2013 Northwest survey by the Oregon Coast National Wildlife Refuge Complex found 7,018 brown pelicans, half the average of the past decade, and the lowest number since 1999.