Release captive-bred individuals into the wild to restore or augment wild populations of owls
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 study in 1985 and 1986 (Henke & Crawford 1987) found three barn owl Tyto alba nests each year in a riverine marsh site in Missouri, USA, with at least 11 eggs being laid and a minimum of seven chicks fledging (complete data are not included). The site was the location of a reintroduction programme in 1983-5 which released 157 owls, and at least two and probably more of the parents at the nests found were captive-bred.Study and other actions tested
A replicated, controlled study in mixed grasslands in Saskatchewan, Canada, in the springs of 1997-2000 and 2002 (Poulin et al. 2006), found that 12 of 26 pairs of burrowing owls Athene cunicularia released together stayed together for the first breeding season, with eight pairs fledging a total of 43 young. Six birds paired with wild owls, raising 31 young in their first years. Reproductive output did not differ between wild and captive pairs, but mortality of released owls was significantly higher than wild birds (19% of 52 released birds dying vs. 4% of 780 wild birds). Five birds (10%) failed to migrate and no released birds returned after migration. Only one offspring from released birds returned to the area the following year, but this was not significantly different from return rates for the offspring of wild birds. This study also describes fostering and release techniques (see â€˜Foster chicks with wild conspecificsâ€™ and â€˜Use holding pens at release sitesâ€™).Study and other actions tested