‘So many questions’: Using decades of data, Boise researcher works to help condors

Every egg counts when it comes to condors.

California condors dwindled to a total population of 22 in the 1980s, and condors lay only one egg every other year.

Thanks to massive conservation efforts that included the work of The Peregrine Fund just south of Boise, condors were brought back from the brink of extinction and now number nearly 400 in the wild, according to the latest condor population report from the U.S. Fish and Wildlife Service.

The Peregrine Fund’s propagation facility at the World Center for Birds of Prey is equipped with a fancy humidity-controlled incubator that automatically rotates eggs, as well as an attentive staff that monitors the eggs daily, as the team there continues its work to aid the condor’s recovery.

But despite their best efforts, each year some eggs fail to hatch.

Carolina Granthon wanted to find out why, and her work could help change the outcomes for the bird.

When Granthon was working on the condor propagation team at the Birds of Prey center, she and her colleagues noticed that “some females are better at breeding than others.” In simple terms, that meant “their eggs are more fertile, or they lay more eggs, or they’re more likely to lay eggs at all.”

Granthon also noticed that some female condors seemed to lay bigger eggs than others, but she wasn’t sure whether size had any relation to the fertility or fate of the egg.

“There’s so many questions I want to answer, we have so many decades of data,” Granthon said.

So she joined The Peregrine Fund research team and enrolled in a Ph.D. program at Boise State University on a quest: What makes a good condor egg?

Decades of data point to one key factor

Granthon gathered data on 499 eggs laid between 1995 and 2024 at the propagation facility. The goal of the study, published in Ornithological Applications in May, was to identify predictors of egg size and fertility in captive condors, and use that knowledge to optimize conservation breeding programs.

Granthon tested the effect of factors such as female age, male age, how long the breeding pair were together, and genetic relatedness.

Egg size did, indeed, turn out to be important. Granthon found that bigger eggs were more likely to be fertile. The overall fertility rate was quite high, with 78% of all laid eggs being fertile. But among the heaviest eggs, the fertility rate was 90%.

And above all else, the most important predictor of egg size and fertility turned out to be female age. Egg size increased from first breeding until around age 10, and then remained constant through middle age. After age 25, egg size began to decline, although some female condors still lay eggs into their 30s and 40s.

The same was true for egg fertility, which increased until 12 years of age, remained constant until about year 30, and then began to decline.

This study marked the first formal report of reproductive senescence, or the decline in reproduction with age, in California condors. Reproductive senescence occurs in many animals, including other birds and even humans, but the timing is different for every species.

The power in putting exact numbers to a condor’s reproductive timeline is that breeding programs can plan ahead. Condors usually take six to eight years to produce their first egg, so to maintain enough breeding pairs of condors, “we need to prep eight years in advance,” Granthon said.

Now that her study has shed light on the importance of condor egg size and fertility declines, researchers can line up breeding pairs well ahead of time.

Patterns are helpful even if the reasons behind them are unclear

One thing that surprised Granthon was that she didn’t identify genetics as a strong predictor of egg size or fertility. Especially because the data backed up one of the common observations made by the propagation team: Some female condors consistently laid bigger eggs than others, year after year.

Given these “striking individual differences among females” that were all raised in the same environment with the same resources, “you would think that they might have some genetic basis,” said Granthon.

Granthon noted that it was difficult to determine the role of genetics because she was working with a “shallow pedigree” due to the long lifespan of condors. The study included nearly 30 years of data, but there were only 26 condors laying eggs throughout that time. Without a larger variety of individual birds across multiple generations, it’s difficult to test the heritability of egg size and fertility, she said.

Even without knowing all the reasons behind egg size, the patterns can be helpful for condor conservation efforts.

Chris McClure, co-author of the study and executive vice president of science and conservation at The Peregrine Fund, said the study “showed that each female tends to lay consistently sized eggs year after year, so a sudden change can be an early warning sign that something is wrong, giving caretakers a chance to step in.”

The future and protection of California condors

Granthon’s work is continuing, with her next research project focusing on what happens after the egg is laid: the factors that determine whether an egg hatches, how the young chick develops, and how the condors fare once released into the wild.

“Beyond condors, the study offers a model for using decades of careful record-keeping to make smarter decisions in efforts to save other endangered species,” McClure said.

Referring to condor breeding programs, Granthon said: “We’ve been doing this for a long time, and we’re going to continue doing this for a long time.”

The Peregrine Fund is not just focused on breeding, but on helping condors once they are in the wild. Lead poisoning continues to be the leading cause of death for the endangered bird, and the agency is a founding partner in the North American Lead-Free Partnership, which collaborates with hunters on conservation efforts and managing lead from hunting ammunition in the food chain.

And in the quest to save America’s largest land bird, Granthon and The Peregrine Fund are not alone. The Condor Recovery Program is a multistate collaboration between nonprofits, zoos, indigenous tribes and federal agencies.