SAN DIEGO -- In the past couple of months, SeaWorld has rescued more than 135 sick brown pelicans. Experts are trying to figure out what is causing the birds' illness.

Geri Smith, from Project Wildlife, has helped rescue the pelicans. In the last month or so, animal rescuers have rescued five to eight times more than the park typically receives. All of the birds are young, under-nourished, and too sick to fly or feed.

Some of the birds have ended up as far away as Arizona, while others have been spotted on the median along Highway 52.

At SeaWorld's Rescue and Rehabilitation Center, the birds are getting the nutrition and care needed to go back to the wild.

"We can't immediately give them food because their digestive tracts can't handle it. So we very slowly get their hydration established and get them onto a gruel until we can get them onto a fish, which is their natural diet," said Dr. St. Leger, a staff veterinarian at SeaWorld, said.

The birds are being tested for viruses and toxins but early indications show its likely there is just not enough food.

"Is it because there are less fish? Or the fish are somewhere else? Or they can't get at the fish? We're actually inquiring at the southwest fisheries to find out what's happening with the anchovy and sardine stock in San Diego County," St. Leger told 10News.

About 40 of the rescued pelicans have died so far. The ultimate goal is to return the birds to the wild.

Animal rescuers urged civilians to not approach sick pelicans, but instead call rescue experts and they will get the birds the care they need.