Every egg matters when you’re trying to save a critically endangered bird from extinction.

That’s especially true for California condors, which, with their 10-foot wingspan, are North America’s largest birds. Condors nearly went extinct in the 1980s as a result of hunting, lead contamination, DDT poisoning, and other factors. The last 22 California condors were brought into captivity in 1987 in a last-ditch effort to breed them in safety and save the species from disappearing.

That desperate move was a success. Today, the California condor population has risen to more than 435—all descended from just 14 breeding individuals—and the birds have returned to the skies above California, Arizona, Utah, and Baja, Mexico.

That number would be much higher, however, if every California condor egg hatched a healthy young chick. It isn’t always the case. Eggs fail in captivity and in the wild for a number of reasons, but one of the most worrying is a rare genetic condition called chondrodystrophy, a lethal form of dwarfism.

(Photo: David McNew/Getty Images)

“The chicks born with this disease show very short extremities,” said Cynthia Steiner, acting associate director of genetics at San Diego Zoo Global, who is studying the disease. “It produces late embryonic mortality. The chicks die right before or right after hatching.”

Chondrodystrophy first showed up in five California condor eggs in the late 1990s, a period when the population was just starting to climb again. “It was a big alarm for the managers of the captive population,” Steiner said. Since then the problem has appeared several other times, although exactly how often is unclear because not every failed egg is fully examined to determine why it did not hatch successfully. Steiner said the data that researchers do have suggest that some kind of malformation and embryonic death affected about 120 fertile eggs over the past 22 years.

Conservationists have a pretty good idea which California condors carry the recessive genes that cause chondrodystrophy, but proving that—and managing the problem—remains difficult. “We have a preliminary test,” Steiner said. “It works, it’s useful, but it’s not 100 percent accurate.”

Steiner is working to change that. She and her team are in the process of identifying the exact genetic cause of the disease. “By identifying the genetic basis, we’re going to be able to design a better test for identifying the carriers,” she said. The work was recently the recipient of a $53,000 grant from the Morris Animal Foundation.

Steiner said the genetic work she and her colleagues are doing “is kind of unprecedented. For endangered species, this is brand-new.”

One of the things that made the work possible is the same thing that caused this problem in California condors in the first place: their small founding population—remember, they’re all descended from just 14 individuals—and the resulting genetic bottleneck. That lack of genetic variety allows recessive genes like the ones that cause chondrodystrophy to spread through the population, but it also makes the entire species easier to study genetically.

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That recently resulted in the sequencing of the entire California condor genome. To accomplish this, researchers turned to San Diego Zoo’s Frozen Zoo, which contains more than 10,000 genetic samples from nearly 1,000 species. Included in that collection are samples from nearly 600 California condors.

They didn’t need to test all 600 of those samples, Steiner explained. They had samples from the original 14 founders, along with 22 key individuals. Those 36 birds, Steiner said, provided them with a good genetic record for the entire living population.

Similar work is being done with a handful of other endangered species, but it’s much harder to accomplish without such a clearly defined founder population. For example, conservationists are sequencing the genomes for every living kakapo, a critically endangered flightless parrot in New Zealand. There are only about 150 kakapos on the planet, and they also suffer from genetic disorders, said Andrew Digby, science adviser for Kakapo Recovery.

“Having the genomes of all individuals will enable us to accurately assess correlations of sperm quality, egg fertility, embryo death, and hatching success with inbreeding, and to identify reproductive markers,” he said. Learning more about the birds’ genetics will help the scientists to manage the species, particularly with artificial insemination. They also hope to learn about the genetic basis for disease immunity, an essential tool in such a small population, as well as more about how aging affects kakapos, as the ages of about 25 percent of the living birds are not known.

Digby said he believes the approach “is certainly applicable to other endangered species, and particularly those with low reproductive success and disease issues.” The California condor work is more specific, and the genetic tests would not be of value to other species, but Steiner said the methodology would be useful for other species facing similar challenges.

Steiner said the most valuable part of the work is that it’s not theoretical. “The application of all of this genetic work is in assistance of our main goal: the conservation of the California condor,” she said.