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 Niño, but that isn't expected to begin until this summer and the drop also was much steeper.
"During most El Niño 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.