In the first comprehensive assessment of the reproductive rates of critically endangered California Condors (Gymnogyps californianus) recovering from complete extirpation in the wild, we analyzed 20 years (1999–2018) of data from condor flocks in southern and central California. We found that several anthropogenic threats affected reproductive rates: (1) coastal space use by female condors was associated with lower hatch probability, presumably due to marine mammal foraging and associated DDE exposure, (2) trash ingestion by chicks decreased fledging probability prior to implementation of trash management in 2007, and (3) all parent deaths during rearing resulted in chick or early fledgling deaths, and most parental deaths were due to lead poisoning. We also detected several effects on reproductive rates of the complex individual-based management of condors, which involves ongoing releases of captive-bred individuals and health interventions including treatment of lead poisoning. Recruitment rates were lower for new release sites, which we attribute to a lack of individual- and flock-level experience. In addition, the number of free-flying days in the wild was positively associated with female and male recruitment and with female rebreeding probabilities, indicating that removing individuals from the wild may reduce their breeding success. Finally, probabilities of recruitment, rebreeding, and fledging all increased with age, and given the age distribution skew of the recovering flocks towards younger individuals, overall reproductive success was lower than would be expected at the stable age distribution. Thus, reproductive rates should increase over time as the mean age of California Condors increases if current and emerging threats to reproduction, including the loss of breeders due to lead poisoning, can be addressed.

Observations consisted of near-daily attempts to locate each individual visually or by telemetry signal. Starting in 2002 in the southern flock and 2008 in the central flock, observations at cameras established at proffered feeding stations were also recorded and included in our data. Given the intensive monitoring, we believe that essentially all breeding attempts that produced an egg were detected. Nests were monitored visually and nesting phenology was inferred based on parental behavior, or where possible, observations of eggs and chicks, including regular nest entries at most nests. Starting in 2012, webcams were installed at a subset of remote nests. Additional details on the collection of daily condor data are available through the U.S. Fish and Wildlife Service (https://ecos.fws.gov/ecp/species/8193). Data are summarized as described in the associated paper, “Anthropogenic threats and young age skew lower fertility in the first two decades of reintroduction for critically endangered California Condors in California”

Data and code compiled in R.
R Core Team (2022). R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria. URL https://www.R-project.org/.