Action: Release captive-bred individuals into the wild to restore or augment wild populations of rails
Key messagesRead our guidance on Key messages before continuing
- One replicated study from Australia found that released Lord Howe Island woodhens Tricholimnas sylvestris successfully bred in the wild, re-establishing a wild population.
- A replicated study from the UK found high survival of released corncrake Crex crex in the first summer (although no data were available on overwinter survival or breeding).
- A replicated study in New Zealand found very low survival of North Island weka Gallirallus australis greyi following release, mainly due to predation by invasive mammals.
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 species’ former range, or to augment an existing population.
Release techniques vary considerably, from ‘hard releases’ involving the simple release of individuals into the wild to ‘soft releases’ 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 (‘Use holding pens at release sites’, ‘Use ‘anti-predator training’ to improve survival after release’ 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 ‘Control mammalian predators on islands’). 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 ‘Use captive breeding to increase or maintain populations’.
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.
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 ‘Translocate individuals’.
- Miller B. & Mullette K.J. (1985) Rehabilitation of an endangered Australian bird: the Lord Howe Island woodhen Tricholimnas sylvestris (Sclater). Biological Conservation, 34, 55-95
- Bramley G.N. & Veltman C.J. (1998) Failure of translocated, captive-bred North Island weka Gallirallus australis greyi to establish a new population. Bird Conservation International, 8, 195-204
- Carter I. & Newbery P. (2004) Reintroduction as a tool for population recovery of farmland birds. Ibis, 146, 221-229