Population reinforcement and demographic changes needed to stabilise the population of a migratory vulture.
Abstract
One approach to stabilise small and declining populations is to breed individuals in captivity and release them into the wild to reinforce existing populations while working to reduce threats. Population reinforcement programmes require long-term commitments to be successful and can divert limited resources from other conservation measures. A rigorous evaluation whether reinforcement can stabilise a population is therefore essential to justify investments. Many migratory species incur high mortality during their first migration, and releasing captive-bred birds at an older age may therefore benefit reinforcement programmes for migratory birds. We examine whether a small and declining population of a long-distance migratory raptor-the Egyptian Vulture Neophron percnopterus-can be stabilised using population reinforcement that reduces mortality during the first migration. We used an integrated population model to evaluate realistic reinforcement and survival improvement scenarios to estimate how many captive-bred birds would need to be released to stabilise the population. Survival probability of wild juveniles during their first year (0.296; 95% CI 0.234-0.384) was too low for a stable population (population growth rate 0.949; 95% CI 0.940-0.956), but captive-bred juveniles released in their second calendar year had improved survival (0.566; 95% CI 0.265-0.862) during their first year in the wild. Reinforcement of 15 birds per year for 30 years was insufficient to achieve a neutral or positive population growth rate. However, reinforcement reduced the probability of extinction by 2049 from 48% without reinforcement to <1% if 12 or more birds were released every year for 30 years. A 6% increase in annual survival probability would likely lead to a stable population without any reinforcement. Synthesis and applications. Although releasing captive-bred birds can reduce high juvenile mortality during first migration and assist in postponing local extinction, further improvements of survival in the wild are required to safeguard a migratory population where threats in the wild will persist for decades despite management.