Survival rates of a reintroduced population of the Crested Ibis Nipponia nippon in Ningshan County (Shaanxi, China)

A reintroduction project for the endangered Crested Ibis Nipponia nippon was conducted in Ningshan County (Shaanxi Province) on May 31, 2007. Post-release monitoring of the survival of the reintroduced population was carried out extensively from 2008 to 2015. Data collected over eight years after release were used to estimate the annual survival rate for different cohorts using a Cormack-Jolly-Seber model with capture-recapture data. The mean annual survival rates for all individuals were estimated to be 0.738 (95% CI: 0.547-0.801) and 0.752 (95% CI: 0.478-0.887) for released birds. For different age classes in the recipient population, the survival rates were estimated to be 0.384 (95% CI: 0.277-0.504), 0.853 (95% CI: 0.406-0.978), and 0.812 (95% CI: 0.389-0.950) for yearlings, juveniles and adults, respectively. The higher mortality for yearlings has greatly decreased the survival rate and our focal population was indeed sensitive to changes in yearling survival. Therefore, effective protection of yearlings was crucial to population persistence, as well as to juveniles and adults. The large proportion of mature individuals in our focal population indicated a gradually growing population. There was a slight bias towards males in the adult sex ratio with the increase of wild-born offspring, but it was not statistically significant. Therefore, we conclude that the primary goal of establishing a self-sustaining population of the Crested Ibis in part of their historical range has been achieved. Finally, we discuss factors affecting the survival of the reintroduced population and we propose some changes for future management of endangered species.