Which factors affect post-release settlement of Crested Ibis Nipponia nippon on Sado Island, Japan?

Abstract

Approximately one-third of all conservation translocations have failed to establish a self-sustaining population. Despite this historically low success rate, reintroduction is increasingly used in species recovery programmes in accordance with IUCN guidelines. Two commonly used methods of reintroduction involve ‘hard’ or ‘soft’ releases. A ‘hard’ release means immediate release from captivity into the wild, whereas ‘soft’ release involves individuals being kept for a period at a pre-release site and/or an extended period under care at the release site. In general, in recent reintroduction programmes, soft release is preferred over hard release because studies have shown that soft release can increase translocation success by encouraging animals to stay near the release point thereby utilizing supplementary food and delaying dispersal. The Crested Ibis Nipponia nippon used to be widespread in Japan, but became extinct in the wild during the early 1980s. In order to re-establish a wild population, a reintroduction programme has been implemented on Sado Island since 2003. Ten ibises were hard-released in 2008, and 20 were soft-released in 2009. In this paper, we quantify the differences in post-release movements by comparing the results of 2008 and 2009 releases, to determine whether release methods influence post-release behaviour. Linear mixed-effects modelling indicates that the post-release movements of birds varied depending on their gender, style of release, and season. In the reintroduction programme for the Crested Ibis on Sado Island, Japan, soft release appears to encourage birds to remain near the release site and to form a flock immediately after release.