The Crested Ibis Nipponia nippon became extinct in the wild in Japan in 1981, but was reintroduced on Sado Island in 2008, where the population has reproduced successfully since 2012. A total of 157 ibises is estimated to reside on Sado Island. To facilitate monitoring of survival and mortality, all the released individuals were individually identified through different combinations of color rings, and a uniquely numbered ring. However, because they were overlooked or present in inaccessible nests, not all wild-hatched nestlings were ringed for identification. In addition, the detection rate of the marked individuals might decline with an increase in the population size owing to the limited monitoring effort. Ministry of the Environment (MOE) has determined the number of unmarked individuals by the maximum number of simultaneous observations, and the number of marked individuals has been counted by subtracting the number of individuals that have not been observed for six months. Under such circumstances, the population size could be underestimated. To address this shortcoming, we fitted a mark-resight logit-normal model for estimating unmarked individuals, and a poisson-log model with robust design for estimating total population size including marked individuals, using program MARK. We also compared estimated numbers with observed ones. There were 8.48 unmarked individuals since the first successful reproduction in 2012. The estimated number reached a peak of 30.23 in 2014, but has gradually decreased to 20.30. In all study periods, the estimated number of unmarked individuals was larger than observed ones. However, the estimated number of marked individuals was almost the same as the observed one, but the numbers published by MOE were overestimations. At the end of September 2015, there were approximately 109 released and 41 wild-hatched individuals on Sado Island. When evaluating the reintroduction program, we recommend that the population size of the Crested Ibis should be estimated using mark-resight models. The result of this study will also contribute towards increasing accuracy in population estimates in other reintroduction programs.
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