Action: Release birds as adults or sub-adults, not juveniles
Key messagesRead our guidance on Key messages before continuing
- Three replicated studies found that malleefowl Leipoa ocellata, houbara bustards Chlamydotis undulata macqueenii and cackling geese Branta hutchinsii released as sub-adults, not juveniles had higher survival rates.
- A replicated study from New Zealand found lower survival for black stilts Himantopus novaezelandiae released as sub-adults, compared with juveniles.
- Two replicated studies from Hawaii and Saudi Arabia found lower survival for Hawaiian geese Branta sandvicensis and bustards released as wing-clipped sub-adults, compared with birds released as juveniles.
- Three replicated studies found no differences in survival between ducks, vultures and ibises released at different ages, but a second study of the vulture release programme found that birds released when more than three years old had lower reproductive success than birds released at an earlier stage.
Most captive-bred birds are released as juveniles as adults may have become too conditioned to life in captivity. Under some circumstances, however, adults or sub-adults may survive better.
Supporting evidence from individual studies
A ten-year study of a griffon vulture Gyps fulvus reintroduction programme in river gorges in Aveyron, southern France (Sarrazin et al. 1994) found that annual survival rates were similar or higher for birds released as adults (74% in the first year after release and 98% from the second year onwards, 39 birds released), compared to those released as immature (75% during the first two years after release, 20 birds released). This study is also discussed in ‘Release captive-bred individuals’ and ‘Use education programmes and local engagement to help reduce pressures on species’.
A replicated study in mallee scrub in New South Wales, Australia, in 1987-90 (Priddel & Wheeler 1996) found that survival of released malleefowl Leipoa ocellata was significantly higher for birds released as sub-adults (14-28 months old) than for birds released as juveniles (3-5 months old) (four sub-adults, of 12 released – 33% survived for 36 days; three for at least 428 days and two for at least 787 days vs. all 24 juveniles were dead within 104 days with at least 83% dead within 36 days). The difference in survival was only evident more than eight days after release, with 50% and 42% of sub-adult and juvenile birds surviving the first eight days respectively. At least 21 juveniles (87%) and seven sub-adults (58%) were killed by predators (mainly foxes Vulpes vulpes). Eggs were collected from the wild and artificially incubated and the chicks fed on seeds, mealworms and vegetables before release.
A replicated study over 12-years (Sarrazin et al. 1996) of the same programme as in Sarrazin et al. 1994, found that the nesting success of griffon vultures Gyps fulvus released at the age of three or more (0.42 fledglings/pair for 103 nesting attempts) was significantly lower than that of younger releases and wild-bred birds (0.82 young/pair for 11 attempts). This difference was partially due to lower hatching success for older released birds (55% hatching success for 79 eggs), compared with younger releases and wild-bred birds (75% for 11 eggs). The overall success of the programme is discussed in ‘Release captive-bred individuals into the wild to restore or augment wild populations’.
A 1997 review of the Hawaiian goose (nene) Branta sandvicensis reintroduction programme (Black et al. 1997) concluded that birds released into temporary exclosures before fledging survived better than older birds released in exclosures with their wings clipped. This study is discussed in more detail in ‘Release captive-bred individuals into the wild to restore or augment wild populations’.
A replicated study at a desert steppe site in 1992-4 in southwest Saudi Arabia (Combreau & Smith 1998) found that houbara bustards Chlamydotis undulata macqueenii released as sub-adults had higher survival than those released as chicks, except when released with their wings clipped (48% survival for 59 unclipped sub-adults released in 1992-4 vs. 36% of 14 chicks released in 1993 and 17% of 12 sub-adults with clipped wings released in 1992). Details of the chick releases are discussed in ‘Release birds in ‘coveys’’. This study is also discussed in ‘Release captive-bred individuals’ and ‘Use holding pens at release sites’. The effect of predator removal is discussed in ‘Control predators not on islands’.
A before-and-after study from the Aleutian Islands, Alaska, USA on the cackling goose Branta hutchinsii recovery programme (USFWS 2001) found that releasing young geese was less successful than other strategies (see ‘‘Clip birds’ wings on release’ and ‘Release birds in ‘coveys’). However, the authors note that the release of captive-bred geese was not very successful overall. This study also investigates the effect of Arctic fox Alopex lugopus control on breeding islands (see ‘Predator control on islands’).
A 2007 review of northern bald ibis (waldrapp) Geronticus eremite conservation (Bowden et al. 2007) found no differences in survival between birds released in Israel as breeding adults, juveniles or fledglings. All 56 birds released became emaciated and disorientated and formed poor social bonds. This study is also discussed in ‘Release captive-bred individuals into the wild to restore or augment wild populations’, ‘Use captive breeding to increase or maintain populations’, ‘Artificially incubate and hand-rear birds in captivity’, ‘Use holding pens at release sites, ‘Clip birds’ wings on release’, ‘Use microlites to help birds migrate’ and ‘Foster birds with non-conspecifics’.
A review of black stilt (kaki) Himantopus novaezelandiae releases in South Island, New Zealand, between 1993 and 2005 (van Heezik et al. 2009) found that 20% of 150 birds released as juveniles (60-90 days old) and 13% of those released as sub-adults (nine months old) were alive two years after release (with 25 juvenile releases and 52 sub-adults not yet at breeding age). Neither group was more likely to be seen at the release site. Eggs came from both wild and captive birds and were artificially-incubated until hatching. This study is discussed in more detail in ‘Release captive-bred individuals’ and ‘Release birds in groups’.
- Sarrazin F., Bagnolini C., Pinna J.L., Danchin E. & Clobert J. (1994) High survival estimates of griffon vultures (Gyps fulvus fulvus) in a reintroduced population. The Auk, 111, 853-862
- Priddel D. & Wheeler R. (1996) Effect of age at release on the susceptibility of captive-reared malleefowl Leipoa ocellata to predation by the introduced fox Vulpes vulpes. Emu, 96, 32-41
- Sarrazin F., Bagnolini C., Pinna J.L. & Danchin E. (1996) Breeding biology during establishment of a reintroduced griffon vulture Gyps fulvus population. Ibis, 138, 315-325
- Black J.M., Marshall A.P., Gilburn A., Santos N., Hoshide H., Medeiros J., Mello J., Hodges C.N. & Katahira L. (1997) Survival, movements, and breeding of released Hawaiian geese: an assessment of the reintroduction program. The Journal of Wildlife Management, 61, 1161-1173
- Combreau O. & Smith T.R. (1998) Release techniques and predation in the introduction of houbara bustards in Saudi Arabia. Biological Conservation, 84, 147-155
- (2001) Aleutian Canada goose road to recovery. U.S. Fish & Wildlife Service report.
- Bowden C.G.R., Boehm C., Jordan M.J.R. & Smith K.W. (2007) Why is reintroduction of northern bald ibis Geronticus eremita so complicated? An overview of recent progress and potential. Pages 27-35 in: Toronto, Onatario, Canada.
- van Heezik Y., Maloney R.F. & Seddon P.J. (2009) Movements of translocated captive-bred and released Critically Endangered kaki (black stilts) Himantopus novaezelandiae and the value of long-term post-release monitoring. Oryx, 43, 639-647