Action Synopsis: Bird Conservation About Actions

Release captive-bred individuals into the wild to restore or augment wild populations of raptors

How is the evidence assessed?
  • Effectiveness
  • Certainty
  • Harms

Study locations

Key messages

  • Five studies of three release programmes from across the world found the establishment or increase of wild populations of falcons Falco spp.
  • Five studies from the USA found high survival of released raptors (with between one and 204 birds released), whilst two found that released birds behaved normally and hunted successfully.
  • One study from Australia found that a wedge-tailed eagle Aquila audax had to be taken back into captivity after acting aggressively towards humans, whilst another Australian study found that only one of 15 brown goshawks Accipiter fasciatus released was recovered, although the authors do not draw conclusions about survival rates from this.


About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

  1. A small, pre-1980, study in a national park in the Australian Capital Territory, Australia (Olsen & Olsen 1980), found that a female wedge-tailed eagle Aquila audax released into a 5,500 ha nature reserve successfully adapted to release and began hunting European rabbits Oryctolagus cuniculus. However, the bird had to be recaptured after it attacked people entering her hunting area, two months after release. The eagle came from Melbourne Zoo and was fearful of humans both before release and after recapture.

    Study and other actions tested
  2. A replicated study from the eastern USA between 1975 and 1979 (Barclay 1980) found that 72% of 204 captive-bred peregrine falcons Falco peregrinus that were hacked in artificial and natural sites survived to independence, with three groups of releases being ‘adopted’ by wild adults. Success was higher for birds released at artificial sites (i.e. from a tower), compared to natural sites (i.e. from cliffs), mainly because of high rates of predation by great horned owls Bubo virginianus at cliffs. Most birds stayed in the release area and first year survival appears comparable with wild birds. In 1979, three pairs consisting of released birds were known.

    Study and other actions tested
  3. A replicated study of bald eagle Haliaeetus leucocephalus reintroductions from a breeding centre in Maryland, USA (Wiemeyer 1981), found that all eleven captive-bred, parent-reared birds hacked at two sites in New York and Georgia, USA, successfully reached independence. This study is also discussed in ‘Use captive breeding to increase or maintain populations’, ‘Use artificial insemination in captive breeding’ and ‘Foster birds with wild conspecifics’.

    Study and other actions tested
  4. A replicated study from Canberra, Australia (Olsen & Olsen 1981), found that, only 1 of 15 captive-bred brown goshawk Accipiter fasciatus chicks released into a suburban habitat between 1976 and 1979 was recovered: a male hit by a car 960 km away and nine months after release. The authors note that all young were very secretive after release. Young were hacked by being fed for between two weeks and two months after release. This study is also discussed in ‘Use captive breeding to increase or maintain populations’.

    Study and other actions tested
  5. A study from wetlands in Kentucky and Tennessee, USA (Altman 1983), describes the successful release, through hacking, of a captive-bred, juvenile bald eagle Haliaeetus leucocephalus in summer 1981. The eagle was fed in an enclosure until 14.5 weeks old and began flying immediately after it was released. The eagle remained close to the release site for 39 days, hunted successfully (with a 50% success rate) and appeared to behave normally until it dispersed from the study area.

    Study and other actions tested
  6. A replicated 1993 study (Cade & Jones 1993) found that 77% of 164 captive-bred and raised Mauritius kestrels Falco punctatus released into the wild in tropical forests in southern Mauritius between 1986 and 1992 survived until independence. Release involved hacking on an offshore island for several weeks before being released on the mainland. Before the release of captive-bred individuals, the wild population had grown from five individuals in 1973 to 31 in 1986. Following fostering (see ‘Foster chicks or eggs with wild conspecifics’) and releases, the wild population reached at least 30 breeding pairs in 1991-2. This study is also discussed in ‘Use captive breeding to increase or maintain populations’, ‘Artificially incubate and hand-rear birds in captivity’.

    Study and other actions tested
  7. A 1995 update (Jones et al. 1995) of the same conservation programme studied in (6), found that hacking released captive-bred Mauritius kestrels Falco punctatus nestlings significantly contributed to the recovery of the natural population. A total of 331 birds were released into various sites from 1984-1985 and 1993-1994 of which 78% became independent and 61% survived their first winter. Of 208 fledglings hacked (25-34 day old nestlings were put into small groups in a nest box and food was provided while hunting skills were honed), 79% became independent. Most moved out of the release area between 85-100 days; similar to that of natural parent-raised birds. However, only 38% of released first-year females successfully fledged young whereas older females averaged 2 fledglings/nest (from 64% of nests). The remainder of the released captive-bred young were fostered (see ‘Foster eggs or chicks with wild conspecifics’). At the end of the 1993-1994 breeding season, the natural population had recovered to 222-286 birds (containing at least 56 breeding pairs and 40-70 non-breeding birds), from a low of four wild individuals.

    Study and other actions tested
  8. A replicated study in 1993-4 (Perez & Zwank 1996) found that four-week survival rates of captive-bred aplomado falcons Falco femoralis hacked at a wetland site in southern Texas, USA, ranged from 58% (five known mortalities from 12 birds released in 1994) to 85% (four known mortalities from 26 birds released in 1993). Predation by great horned owls Bubo virginianus and coyotes were the main causes of mortality. Released birds had larger range sizes than predicted, which the authors suggest is due to birds having expanded ranges before pairing up. Birds were transported to the release site when four weeks old and fed there before being released at 37 days old. Food was then provided until birds no longer returned to feed.

    Study and other actions tested
  9. A replicated study of a captive-release programme in eastern Germany (Kirmse 2001) found that the release of 201 captive-reared peregrine falcons Falco peregrinus between 1990 and 2000 led to a population of at least 22 adult peregrines in the study area in 2000. This population of peregrines was unique in that it nested in trees, and imprinting techniques meant that all but 11 released birds nested in trees. The remaining 11 used buildings or cliffs, and none of their offspring reverted to tree-nesting.

    Study and other actions tested
  10. A study in the summers of 1999-2000 at a river cliff site in Iowa, USA (Powell et al. 2002), found that two week survival of 38 (21 in 1999, 17 in 2000) juvenile peregrine falcons Falco peregrinus released through hacking was between 74% and 89%, with overall weekly survival estimated at 98.8%. Movement away from the release site was higher in 2000, possibly due to the large numbers of great horned owls Bubo virginianus seen in the area (although no mortalities were due to owl predation).

    Study and other actions tested
  11. Observations in Panama of two captive-bred harpy eagles Harpia harpyja, indicated that after release, prey diversity collected and predation rates were broadly consistent with that of wild birds (Touchton et al. 2002). In 1998 in captive breeding facilities in USA, two harpy eagles were hatched and reared using puppets (to avoid imprinting on human carers, see ‘Use puppets to increase the survival or growth of hand-reared chicks’ for studies on this intervention), then placed in an enclosure with an adult female eagle. Near fledging (161-165 days old) they were transferred to an aviary at a release site in Panama, where they were habituated for 4-5 weeks prior to release. They were provided with supplementary food until they ceased to visit (11 months). Both birds were recaptured and relocated to a nearby safer site, the male re-released on 16 June and the female on 10 October, 1999. The eagles were monitored during June 1999 to August 2000. Both made captures of wild prey with apparent ease, despite lack of human training or parental guidance.

    Study and other actions tested
  12. A replicated study in Texas, USA, between 1993 and 2002 (Brown et al. 2004), found that all of the 154 northern aplomado falcons Falco femoralis septentrionalis studied displayed hunting behaviour without having been taught it. Birds were hacked from 22 sites in groups of between two and eight birds, taken to the hacking site at 30 days old, released at 38-41 days old and provided with food for a further six weeks. Males began hunting earlier (19 days after release for 78 birds vs. 24 days after release for 76 birds), but made their first kills later (35 days after release for 19 kills vs. 32 days after release for 19 kills by females). Group hunting was also observed.

    Study and other actions tested
  13. A 2004 review (Nicoll et al. 2004) of the same Mauritius kestrel Falco punctatus release programme as in Cade & Jones 1993, between 1987 and 2001  found that survival estimates for adult kestrels were similar, irrespective of whether they were ‘hacked’ as fledglings (and provided with supplementary food until independence), or fostered to wild breeding pairs or wild-bred (80% for 42 fostered birds; 80% for 46 hacked birds and 75% for 284 wild-bred birds). Survival estimates for juvenile kestrels were far more variable, but did not appear to differ between treatments (36-72% for hacked, 23-100% for fostered and 31-80% for wild-bred). A total of 40 breeding pairs were monitored in 2000-1. Overall, the wild kestrel population across Mauritius reached an estimated 500 to 800 individuals in 2000, compared to five individuals in 1973.

    Study and other actions tested
  14. A review of a reintroduction programme for northern aplomado falcons Falco femoralis septentrionalis in coastal plains in Texas, USA (Brown et al. 2006), found that the release of captive-bred falcons since 1993 had led to the establishment, by 2002-4, of 38 breeding pairs in the two study areas. During 2001-3, 141 captive-bred falcons reached independence in the study area, and 75 chicks fledged. Of these, 43 (19 released and 24 wild-bred) were seen after fledging at least once. Of 18 birds recruited into the breeding population (i.e. forming breeding pairs), only three (17%) were captive-bred and released and 15 (83%) were wild-bred. Captive-bred birds were ‘hacked’ during release. This involved providing cohorts of 2-8 birds with food for 21 days after release. Those birds seen after 21 days were said to have reached independence.

    Study and other actions tested
Please cite as:

Williams, D.R., Child, M.F., Dicks, L.V., Ockendon, N., Pople, R.G., Showler, D.A., Walsh, J.C., zu Ermgassen, E.K.H.J. & Sutherland, W.J. (2020) Bird Conservation. Pages 137-281 in: W.J. Sutherland, L.V. Dicks, S.O. Petrovan & R.K. Smith (eds) What Works in Conservation 2020. Open Book Publishers, Cambridge, UK.


Where has this evidence come from?

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Bird Conservation

This Action forms part of the Action Synopsis:

Bird Conservation
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