Statistical model correctly predicts nest success for the critically endangered California Condor

California Condors (Gymnogyps californianus) raise their young in crazy places. Cliffsides or rockfall on the sides of mountains are the typical haunts of condor families, making nest monitoring difficult for even the most intrepid field biologists to achieve. A new paper titled “Modeling Individual-level and Population-level Nest Success of California Condors from Movement Data,” published in the Journal of Raptor Research, demonstrates the efficacy of using statistical modeling paired with GPS tracking to remotely determine whether condor nests are successful. The methods featured in this study proved correct in assessing nest fate for 63 out of 65 nests, an accuracy of 97%. The species is expanding its range and nesting further afield from condor management hubs, often on private or inaccessible land. For a species still dependent on human management, tools like these could prove highly useful in keeping a close eye on the health of wild populations and better informing conservation decisions. Additionally, these methods could be applicable for other tracked wildlife species that are difficult to monitor in the field.

The California Condor is a large vulture weighing up to 20 pounds with a wingspan of nine feet. They have endured a plethora of human-caused challenges including hunting, habitat modification, microtrash ingestion, DDE poisoning, and most deleterious, consistent lead poisoning. By the year 1982 only 22 birds remained in the wild. Following a rigorous breeding and reintroduction program, the first captive-bred individual was released in Ventura, California in 1992, and today, there are over 350 free-flying condors. Each flock is managed by one or multiple agencies, and captive-reared individuals are released each year to sustain wild populations, in part because lead poisoning continues to harm wild condors across their range.

Lead authors of the paper, Dr. Andrea Blackburn of the Santa Barbara Zoo and Dr. Joseph Eisaguirre, of the US Geological Survey’s Alaska Science Center, with support from the US Fish and Wildlife Service, Santa Barbara Zoo, USGS Ecosystems Mission Area, and other organizations, adopted a model previously used on Golden Eagles to assess California Condor nesting success from 2015 through 2022. With tracking data, they coded movements of the birds and fed it to the model, which remotely determined whether a condor pair was attending to a nest and for how long. The model’s accuracy hinged on its ability to detect changes in space use and movement behaviors of individual birds. The model’s results were corroborated with field observations and camera footage to determine accuracy, which was extremely high, and allowed the team to comfortably conclude nest success rates for individual pairs as well as the Southern California population as a whole, across a span of eight years.

In the future, the team intends to introduce new factors into their model, like the age of the parents or the protection status of a pair’s chosen nesting area since condors are known to face higher risks of lead poisoning outside of protected areas. Blackburn says they also hope to introduce factors that will “increase understanding of how management actions, like proffering food, affect condor nesting outcomes.” She adds that they aim to use this model to “reduce on-the-ground presence and therefore minimize disruption for condors and landowners.”

California Condors are scavenging raptors and can therefore function as bioindicators for the health of their environment as they encounter a wide range of carcass types and, subsequently, ingest toxic substances such as lead from spent hunting ammunition and DDE pesticides. If conservation actions lead to self-sustaining condor populations in Southern California and other parts of their range, those communities will regain a valuable member of their wildlife tapestry from both ecological and cultural lenses. With all that the condor has been through, this is a shared hope by many agencies across North America.

###

Paper

Blackburn, et al. (2025). Modeling Individual-level and Population-level Nest Success of California Condors from Movement Data. Journal of Raptor Research, 59(3): 1-11.
DOI: 10.3356/JRR-2464

Notes to Editor:

1. The Journal of Raptor Research (JRR) is an international scientific journal dedicated entirely to the dissemination of information about birds of prey. Established in 1967, JRR has published peer-reviewed research on raptor ecology, behavior, life history, conservation, and techniques. JRR is available quarterly to members in electronic and paper format.
 

2. The Raptor Research Foundation (RRF) is the world’s largest professional society for raptor researchers and conservationists. Founded in 1966 as a non-profit organization, our primary goal is the accumulation and dissemination of scientific information about raptors. The Foundation organizes annual scientific conferences and provides competitive grants & awards for student researchers & conservationists. The Foundation also provides support & networking opportunities for students & early career raptor researchers.