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.

Stage structured prey-predator model incorporating mortal peril consequential to inefficiency and habitat complexity in juvenile hunting

“…Owing to inefficiency, many migratory bird species incur high mortality of juveniles during their first migration, this dilemma could be curtailed by releasing captive-bred birds at an older age in the subsequent season. Attributable to the dwindling population size of the Egyptian Vulture, this reinforcement has become a necessity to avoid their extinction [46] . On the other hand, the juveniles becoming accustomed to human interference could have either a positive or negative ramification during their handling of a prey with an anti-predator behaviour [41] .…”

Potential importance of wind patterns and guidance opportunities for the conservation of the Egyptian Vulture Neophron percnopterus in Italy

“…the central flyway, were included in the "eastern" group). iii) captive-bred juveniles released in the Balkans in their second calendar year, have shown higher survival probability than those released in their first calendar year, as well as than wild-hatched juveniles (Oppel et al 2021c). Interestingly, unlike birds released in the Balkans, a recent study has shown no difference in apparent survival between captive-bred and wild birds in the Egyptian Vulture population in Israel (Efrat et al 2022).…”

“…In studies aiming at investigating differential survival between Egyptian Vulture populations throughout the full annual cycle (Buechley et al 2021, Oppel et al 2021c) it was found that: i) lower survival mostly occurred during migratory periods and; ii) there were proportionally fewer confirmed mortalities in the western flyway compared to the central and eastern one (in Buechley et al 2021, individuals from Italy, i.e. the central flyway, were included in the "eastern" group).…”

“…It is also recommended to release captive-bred individuals earlier in the season, rather than at the end of it in late August, by using 2 st calendar year birds. In the case of particular technical circumstances that still require releases in Southern continental Italy (Apulia, Basilicata and Calabria), it is still recommended to release 2 st year birds as a priority as they are expected to have higher survival probability (Oppel et al 2021c) and to migrate earlier in the season (compared to fledging young), thus increasing their chances of meeting older birds. Indeed, using delayed release, captive-bred individuals can be an effective approach for successful restocking programmes (e.g.…”

Evaluation of ongoing Crested Ibis (Nipponia nippon) reintroduction using an integrated population model and Bayesian population viability analysis

“…In addition, IPM can be used to conduct PVA by simply extending the time horizon to obtain predictions (Oppel et al . 2014, 2021, Saunders et al . 2018, Schaub & Kéry 2021).…”