Release captive-bred individuals into the wild to restore or augment wild populations of storks and ibises
Overall effectiveness category Unknown effectiveness (limited evidence)
Number of studies: 2
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 in southeast Turkey in 1977-88 (AkÃ§akaya 1990) found that northern bald ibis Geronticus eremita bred or kept in captivity did not increase the Turkish population. From 1981-88, 67 individuals were released and 12 (18%) migrated with the wild population (note: this excludes 1984, 1986 and 1987, for which data were not available). There was high winter mortality among birds that did not migrate and also high mortality on migration. The wild population in the study area declined over the study period, with five pairs in 1986, seven in 1987 and only four birds returning in 1988. This study is also described in â€˜Use captive breeding to increase or maintain populationsâ€™ and â€˜Provide artificial nesting sitesâ€™.Study and other actions tested
A 2007 review of northern bald ibis (waldrapp) Geronticus eremita conservation (Bowden et al. 2007) found varying success in release programmes, dependent on the techniques used. Trials in Israel using a variety of techniques (described in â€˜Release birds as adults or sub-adults, not juvenilesâ€™ and â€˜Clip birdsâ€™ wings on releaseâ€™) found that all 56 birds released became emaciated and disorientated and formed poor social bonds. Similarly, the release of 73 birds in Spain between 2004 and 2006 has not resulted in the formation of a stable colony. However, the release of 43 birds in Austria has led to the establishment of a colony in the wild. This study is also discussed in â€˜Use captive breeding to increase or maintain populationsâ€™, â€˜Artificially incubate and hand-rear birds in captivityâ€™, â€˜Use holding pens at release sitesâ€™, â€˜Release birds as adults or sub-adults, not juvenilesâ€™, â€˜Clip birdsâ€™ wings on releaseâ€™, â€˜Use microlites to help birds migrateâ€™ and â€˜Foster birds with non-conspecificsâ€™.Study and other actions tested