Action: Release captive-bred individuals into the wild to restore or augment wild populations of cranes
Key messagesRead our guidance on Key messages before continuing
- Four studies of five release programmes from the USA and Russia, from a total of eight programmes, found that released cranes had high survival or bred in the wild. Two studies from two release programmes in the USA found low survival of captive-bred eggs fostered to wild birds, compared with wild eggs, or a failure to increase the wild flock size.
- A worldwide review found that releases of migratory species only tended to be successful if birds were released into existing flocks, with higher success for non-migratory populations.
- One study from the USA found that birds released as sub-adults had higher survival than birds cross-fostered to wild birds.
- One study from the USA found that 73% of all mortalities occurred in the first year after release.
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 describing the success of releasing captive-bred Mississippi sandhill cranes Grus canadensis pulla onto a wet pine savanna site in Mississippi, USA (Zwank & Wilson 1987) found that, of 40 birds released between 1979 and 1985, 46% were alive at the end of the study (between one and six years after release). Of the 22 mortalities, 16 (73%) occurred during the first year after release, with three during each of the second and third years. Predation and human-caused mortality were the main causes. Birds were bred in captivity and parent-raised before being rendered temporarily flightless with wing brails and moved to acclimatisation pens. They were between four months and one year old at release.
A replicated study as part of the planning for a whooping crane Grus americana reintroduction programme, a study in Florida, USA, in 1986-7 (Nesbitt & Carpenter 1993) found that greater sandhill cranes Grus canadensis tabida released as sub-adults in a ‘soft release’ programme had higher survival than birds fostered to Florida sandhill cranes G. c. pratensis (56% of 27 birds surviving for one year vs. 39% survival for 34 fostered birds, discussed in ‘Foster eggs or chicks with wild non-conspecifics (cross-fostering)’). The nine to ten month-old cranes were prevented from flying and kept in an open-topped 1.5 ha enclosure for four to six weeks until they were released. Food was provided until the birds no longer returned to the enclosure. Greater sandhill cranes are migratory, whilst Florida sandhill cranes are not. Migratory movements of released birds were larger than a control group of Florida sandhill cranes, but not significantly so.
A replicated study in Idaho, USA, between 1975 and 1991 (Kuyt 1996) found that 215 wild-sourced whooping crane Grus americana eggs that were cross-fostered into sandhill crane G. canadensis nests had higher hatching success than 73 captive-bred whooping crane eggs, fostered at the same time (77% hatching success for wild-sourced eggs vs. 60% for captive-bred). This study is also discussed in ‘Use captive breeding to increase or maintain populations’, ‘Foster eggs or chicks with wild conspecifics’ and ‘Foster eggs or chicks with wild non-conspecifics (cross-fostering)’.
A 1998 review (Davis 1998) found that crane Grus spp. reintroduction programmes have had mixed success, with reintroductions of migratory species generally failing when birds were not released into existing flocks. Reintroductions of Siberian cranes G. leucogeranus has not increased wild flock size, with no reintroduced birds being seen after migration and high mortality during rearing, high poaching levels and few wild birds to guide migration. Releases of semi-wild red-crowned cranes G. japonensis and white-naped cranes G. vipio in southeast Russia found that four of ten released birds migrated successfully, and at least two pairs nested, one successfully. At least 84% of 38 greater sandhill cranes G. canadensis tabida survived for a year after release in Michigan, USA, 74% returned after migration and four males nested. Non-migratory releases generally had higher success: first-year survival of non-migratory whooping cranes G. americana has increased from approximately 34% (1993-4) to 71% (1996), although the population remains very small and may rely on continued releases. Captive-reared Mississippi sandhill cranes G. c. pulla had an overall first-year survival of 70%, an adult survival over 91% following release and in 1992 represented 80% of the wild population. In 1996 there were 13 nesting pairs (the most recorded), with 60% of known pairs having at least one captive-reared individuals. The population, however, remains dependent on releases. The author argues that post-release monitoring is vital to identify causes of mortality.
A replicated, controlled study in a breeding centre in Mississippi, USA, between 1989 and 1996 (Ellis et al. 2000) found that first year survival of captive-bred Mississippi sandhill cranes Grus canadensis pulla was high, with approximately 80% of 132 birds surviving. Birds were released either in mixed flocks (both hand-reared and parent-reared birds) or non-mixed flocks (with just one rearing type). Survival rates over four years were highest for hand-reared birds in mixed flocks (approximately 95% survival for 17 birds), followed by parent-reared birds in mixed flocks (89% of 31 birds), hand-reared in non-mixed flocks (78% of 39 birds) and were lowest in parent-reared, non-mixed flocks (56% of 45). By the end of the study, however, differences between parent and hand-reared birds were no longer statistically significant, although mixed flock birds still had higher survival. Birds were kept in ‘cohorts’ for four to five weeks, before being moved to the release site and kept for a month in uncovered pens before wing brails (which prevent flying) were removed in December. Details of hand-rearing are found in ‘Artificially incubate and hand-rear birds in captivity’.
A replicated study of a whooping crane Grus americana reintroduction programme in 2001-5 in wetlands in Florida, USA (Urbanek et al. 2010), found that winter-releases of this migratory bird proved effective. Average first-year survival of 71 winter-released juvenile birds was 87%, and was higher in later years as techniques improved. Birds were reared by humans wearing costumes (to avoid imprinting on human carers, see ‘Use puppets to increase the survival or growth of hand-reared chicks’ for studies on this intervention) and guided to the release site by an ultralight aircraft. Once there they were kept in holding pens by costumed caretakers. When the habitat prevented this, the juveniles were vulnerable to bobcat Lynx rufus predation, but this problem was overcome by vegetation clearance. Winter releases of this type were advantageous because the intensive care reduced predation by bobcats, juveniles were kept away from harassment by adult birds and juveniles did not lose their fear of humans through contact with tame sandhill cranes G. canadensis. Once released, juveniles showed ordinary migratory and summer behaviour.
- Zwank P.J. & Wilson C.D. (1987) Survival of Captive, Parent-Reared Mississippi Sandhill Cranes Released on a Refuge. Conservation Biology, 1, 165-168
- Nesbitt S.A. & Carpenter J.W. (1993) Survival and Movements of Greater Sandhill Cranes Experimentally Released in Florida. The Journal of Wildlife Management, 57, 673-679
- Kuyt E. (1996) Reproductive manipulation in the whooping crane Grus americana. Bird Conservation International, 6, 3-10
- Davis C. (1998) A review of the success of major crane conservation techniques. Bird Conservation International, 8, 19-29
- Ellis D.H., Gee G.F., Hereford S.G., Olsen G.H., Chisolm T.D., Nicolich J.M., Sullivan K.A., Thomas N.J., Nagendran M. & Hatfield J.S. (2000) Post-release survival of hand-reared and parent-reared Mississippi sandhill cranes. The Condor, 102
- Urbanek R.P., Fondow L.E.A., Zimorski S.E., Wellington M.A. & Nipper M.A. (2010) Winter Release and Management of Reintroduced Migratory Whooping Cranes Grus Americana. Bird Conservation International, 20, 43-54