Egg-laying season has started at four breeding facilities for captive California condors, North America's largest bird.

As of Thursday, the Peregrine Fund's World Center for Birds of Prey near Boise has four eggs among its 16 breeding pairs.

The Oregon Zoo has three eggs, the Los Angeles Zoo four, and the San Diego Zoo Safari Park has two. Egg-laying season can go through early April.

Marti Jenkins, condor propagation manager at the Idaho facility, said geneticists there recommended some changes so there are only 16 rather than 18 breeding pairs this year, and three of those pairs are new together.

"I would say we can expect 16 to 18 eggs," she said. "We're a little unsure."

Condors usually produce only one egg, but workers at the Idaho facility plan to remove some eggs so the breeding pairs produce a second egg. Typically, the eggs are artificially incubated until ready to hatch and then returned to the nest.

Oregon Zoo Senior Keeper of Condors Kelli Walker said the four facilities produce about 40 eggs each year as part of a program to bolster wild populations. All the birds are carefully tracked to maximize genetic diversity.

"We do trade eggs sometimes," Walker said. "We are able to transfer eggs to wild nests if they're required."

California condors weigh more than 25 pounds and have wingspans up to 10 feet. They can live more than 60 years.

The birds teetered on extinction in the early 1980s, prompting conservationists to capture the last 22 in the wild to start a captive breeding program.

Now California condors number more than 400 counting both wild and captive birds. Of the birds in the wild, a 2014 tally found 128 in California, 73 in Arizona and Utah, and 29 in Mexico's Baja peninsula. The rest of the population is in captivity.

Of the 22 original captured condors, 10 are still alive, Walker said. A male referred to as Adult Condor 9 was released back into the wild in 2002 and lives around the Bitter Creek National Wildlife Refuge about 100 miles northwest of Los Angeles, she said.

Another of the original captured birds, Cuayma, is at the Boise site. He was captured as a hatchling in 1983, making him 32 years old.

"He's been a very successful producer of eggs," Jenkins said.

So successful that geneticists removed him from the captive breeding program for the first time this year, and he's being evaluated for release.

"I would be thrilled to see him go back to the wild," Jenkins said.

The Idaho facility has raised more than 200 condors so far that have been released into the wild, she said.