Extreme temperatures impede the release success of captive-bred avian scavengers

Abstract Conservation translocations (for population-reinforcements or reintroductions) are central for managing endangered species. Yet, their implementation is logistically and financially challenging. Because many translocations fail due to the mortality of released individuals, mitigating these events is crucial. The Israeli Gyps fulvus (Griffon Vulture) population is facing local extinction. The recovery efforts include (among other steps) population-reinforcements by releasing translocated or captive-bred individuals into areas where the population has been recently reduced. This study represents a retrospective analysis of a reinforcement effort that, unfortunately, resulted in higher-than-expected mortality, prompting us to investigate mortality causes and improve conservation efforts by preventing future losses. Early survival (the first 10 days) of G. fulvus released (from 2017 to 2020) in the Judean Desert was significantly lower than those released at the other 2 sites (62% compared to 94% and 84%, respectively, n = 61 in total). To understand the factors causing this lower survival, we combined clinical or post-mortem examinations of failed translocations with bio-telemetry. Using GPS and thermometer data, we compared movement, behavior, and weather conditions experienced by released and wild-born individuals active nearby. After excluding several possible pathological factors, we found that compared to wild-born ones, the released individuals in the desert site were exposed to much higher temperatures; had lower flight probability, and shorter daily displacement distances preceding their death (51 °C vs. 40 °C; 0.06 ± 0.11 vs. 0.95 ± 0.02; and 8.44 ± 1.84 km versus 29.63 ± 2.36 km, respectively). These findings suggest heat-stress as the main cause of death, probably because their poorly developed flight ability and inexperience prevented them from seeking shade and cooler areas when experiencing high temperatures. These insights have led to adjustments of the local release protocol (winter release), resulting in a significant improvement (76%) in the early survival of translocated G. fulvus in the Judean Desert, demonstrating how scientific investigation can profoundly improve conservation-translocations.