Senescence and individual variation drive egg size and fertility of captive <i>Gymnogyps californianus</i> (California Condor)

Abstract Conservation breeding programs support biodiversity by supplementing wild populations with captive-bred individuals to bolster population size and genetic diversity. Successful reproduction and long-term viability of offspring, which may depend on early-life traits such as egg size and fertility, are fundamental components of conservation breeding programs but are rarely systematically assessed. We analyzed data from 499 Gymnogyps californianus (California Condor) eggs laid at The World Center for Birds of Prey in Boise, Idaho, USA in 1995–2024 to test the effects of individual-specific traits (age, rearing, inbreeding coefficient, seasonal laying order, and prior-year double laying), pair-specific traits (relatedness and length of pairing), and year effects on fertility and egg weight (a proxy for egg size). Bayesian mixed models suggested that egg weight varied strongly among but not within individual females. Egg weight decreased with seasonal laying order, was independent of prior-year reproductive output, and peaked when females were middle-aged (∼10–25 yr). Egg fertility was high (78%) and showed a similar female age quadratic effect. Egg weight had a positive effect on fertility. We found no effect of any other variables on egg weight or fertility. Our findings highlight senescence and individual variation as factors shaping early-life traits in a captive-bred species. Individuals breeding in captivity have access to unlimited resources, suggesting that variation in egg size is more likely driven by intrinsic female traits than by environmental factors. The genetic and developmental traits that we tested did not mediate this effect on egg size, pointing to other female characteristics as drivers. Optimizing such traits through individualized management could become a valuable tool in conservation breeding and have lasting effects on reproductive success. This study provides a foundation for more systematic, evidence-based decision-making to inform management practices across conservation breeding programs.