Release captive-bred individuals into the wild to restore or augment wild populations of rails
Overall effectiveness category Unknown effectiveness (limited evidence)
Number of studies: 3
Background information and definitions
Captive breeding is normally used to provide individuals which can then be released into the wild to either restore a population in part of the speciesâ€™ former range, or to augment an existing population.
Release techniques vary considerably, from â€˜hard releasesâ€™ involving the simple release of individuals into the wild to â€˜soft releasesâ€™ which involve a variety of adaptation and acclimatisation techniques before release or post-release feeding and care. The following section includes studies describing the overall effects of release projects. Studies that compare specific release techniques are described elsewhere (â€˜Use holding pens at release sitesâ€™, â€˜Use â€˜anti-predator trainingâ€™ to improve survival after releaseâ€™ etc).
Supporting evidence from individual studies
A replicated study on Lord Howe Island, Australia, in 1981-3 (Miller & Mullette 1985) found that captive-bred Lord Howe Island woodhens Tricholimnas sylvestris survived for up to two years in the wild (released after a pig Sus scrufa and goat Capra hircus control programme had been running for several years, see â€˜Control mammalian predators on islandsâ€™). In addition, 19 wild-bred young were reared successfully. Before captive breeding, there were only three pairs known in the wild, which were transferred to captivity. In total, 57 birds were released over three years. This study also describes the captive-breeding efforts, discussed in â€˜Use captive breeding to increase or maintain populationsâ€™.Study and other actions tested
A replicated study on North Island, New Zealand, in 1992-3 (Bramley & Veltman 1998) found that, of 17 North Island weka Gallirallus australis greyi released between October and March at a mixed habitat site, only one bird was alive more than seven months after release. Most individuals were killed by predators (mainly by domestic dogs Canis familiaris) and 12 individuals (71%) survived less than 50 days. Weka had small home ranges (average of 2.7 ha) and dispersed an average of only 1.3 km during the study.Study and other actions tested
A 2004 review of a corncrake Crex crex release programme in a wet grassland site in eastern England (Carter & Newbery 2004) found that only six chicks could be released into the wild in 2002 (due to predation in captivity), and that none was seen in the area the following year. From 140 eggs laid in 2003, 52 chicks were released during summer. Survival was apparently high, but data on overwinter survival and subsequent reproduction were not available. Captive birds were kept in a flock in autumn and winter and then paired off in the spring. Eggs were removed before hatching and incubated artificially. Once hatched, they were hand-fed until they could feed themselves and then released into a pen at the release site when ten days old before being released at 28 days old. This paper also discusses the translocation of red kites Milvus milvus to the UK, discussed in â€˜Translocate individualsâ€™.Study and other actions tested