Action: Release captive-bred individuals into the wild to restore or augment wild populations of songbirds
- A before-and-after study in Mauritius describes the establishment of a population of Mauritius fody Foudia rubra following the release of captive-bred individuals.
- Four studies of three release programmes on Hawaii found high survival of all three species released (Hawaiian crows Corvus hawaiiensis and two thrushes: omao Myadestes obscurus and puaiohi M. palmeri), with the two thrushes successfully breeding. The authors in one note that many of the released puaiohi dispersed from the release site, meaning that repopulating specific areas may require multiple releases.
- A replicated, controlled study from the USA found that San Clemente loggerhead shrike Lanius ludovicianus mearnsi pairs with captive-bred females had lower reproductive success than pairs where both parents were wild-bred.
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 on Hawaii, USA, in 1993-4 (Kuehler et al. 1995) found that at least ten of 12 Hawaiian crows (alala) Corvus hawaiiensis released into the wild survived for at least one month (with three bird surviving at least a year). The status of the other two birds was unknown. Eight of the released birds (including both with unknown statuses) were hand-reared from wild eggs (see ‘Artificially incubate and hand-rear birds in captivity’ for details), the remaining four were captive-bred birds. Birds were transferred to small cages at the release site when 46-63 days old and then into a larger aviary when 62-96 days old. Birds were then slowly released, with the timing dependent on their ability to fly and find food. Supplementary food was provided for several months after release and non-native predators (mongoose Herpestes auropunctatus and black rats Rattus rattus) were trapped from around the aviary whilst releases were on-going (see ‘Invasive and other problematic species’ for more studies of invasive species control).
A replicated study in 1995-6 on Hawaii, USA (Kuehler et al. 2000), found that 80% of 25 captive-bred omao Myadestes obscurus (a thrush) survived for at least 30 days after being released, with at least two chicks being raised. The same study found that 14 (six male, eight female) captive-bred puaiohi Myadestes palmeri (a critically endangered thrush) released at a marshland site on Kaua’i, Hawaii, USA, in 1999 successfully fledged at least seven chicks (from six pairs). Both species were ‘hacked’ by being kept in predator-proof cages at the release site for 6-14 days before release. Food was provided for 17 days after release and predators (feral cats and rats) were poisoned and trapped for 2.5 months before the first puaiohi releases. Details of survival are provided in Tweed et al. 2003. This study is also discussed in ‘Use captive breeding to increase or maintain populations’ and ‘Artificially incubate and hand-rear birds in captivity’.
A replicated study (Tweed et al. 2003) reviewing the same programme as in Kuehler et al. 2000 found that all 14 captive-bred puaiohi Myadestes palmeri released survived for at least 56 days after release. Six of the birds (43%) established breeding territories and two of the remaining females formed pairs with local males. The authors note that repopulating specific areas may require multiple releases because of the 57% dispersal out of the release area.
A continuation of the programme described in Tweed et al. 2003, found that 91% of 21 female and 13 male puaiohi Myadestes palmeri released between 1999 and 2001 survived to independence (defined as 30 days after release) (Tweed et al. 2006). Seventy-five percent of 12 birds monitored for longer survived the next 50 days. All 12 birds (ten female, two male) monitored during the breeding season had active nests, with 31 nests being built over two years by the ten females and 28 becoming active. The fate of 24 nests was known, with 42% fledging at least one young and 38% being predated (probably by rats). Clutch size (average of 2 eggs/nest, 16 nests), daily survival rates (97%) and fledglings/successful nest (1.4 fledglings/nest, ten nests) were similar for released and wild birds, although fewer fledglings/active nest were produced (0.58 fledglings/nest vs. 1.1 fledglings/nest). Release techniques were the same as in (3), but food was provided for up to 30 days.
A controlled, replicated study on San Clemente Island, California, USA, between 2000 and 2006 (Heath et al. 2008) found that pairs of San Clemente loggerhead shrikes Lanius ludovicianus mearnsi with captive-bred females produced fewer fledglings and reared fewer chicks to independence than pairs with wild-bred females (2.6 fledglings/pair and 1.9 independent young/pair for 65 breeding attempts with captive-bred females vs. 3.5 fledglings/pair and 2.6 independent young/pair for 107 attempts with wild-bred females). The same pattern was seen with the origin of the male in a pair, but this was not a significant effect (2.6 fledglings/pair and 1.9 independent young/pair for 54 breeding attempts with captive-bred males vs. 3.6 fledglings/pair and 2.6 independent young/pair for 118 attempts with wild-bred females). Other interventions used are discussed in ‘Control predators on islands’ and ‘Provide supplementary food to increase reproductive success’.
A before-and-after study on Ile aux Aigrettes, Mauritius (Cristinacce et al. 2009), reports that the release of 93 captive-bred Mauritius fodies Foudia rubra in the breeding seasons of 2003-4, 2004-5 and 2005-6 has led to the establishment of a population of 142 individuals and 47 breeding pairs by December 2008. Survival to one year was between 33% (2003-4) and 75% (2005-6), with increases possibly due to the presence of established birds in later years. The first successful breeding was during 2004-5, when five chicks from two females fledged. This increased to 40 from 19 in 2005-6 and 47 from 38 in 2006-7. First-year survival for wild-bred birds was 60-88%. Birds were kept in large aviaries at the release site for at least seven days before release (birds that had not been put in large aviaries before were first placed in small cages within aviaries) and fed a diet of fruit, commercial insectivore food and eggs. Adults were released in groups of one or two (after 30 days in the aviaries), whereas juveniles were released in groups of two to nine birds. Food was provided continuously at the release site.
- Kuehler C., Harrity P., Lieberman A. & Kuhn M. (1995) Reintroduction of hand-reared alala Corvus hawaiiensis in Hawaii. Oryx, 29, 261-266
- Kuehler C., Lieberman A., Oesterle P., Powers A.T., Kuhn M., Snetsinger T.J., Harrity P., Tweed E.J., Fancy S.G., Woodworth B.L. & Telfer T. (2000) Development of restoration techniques for Hawaiian thrushes: Collection of wild eggs, artificial incubation, hand-rearing, captive-breeding, and re-introduction to the wild. Zoo Biology, 19, 263-277
- Tweed E.J., Foster J.T., Woodworth B.L., Oesterle P., Kuehler C., Lieberman A.A., Powers A.T., Whitaker K., Monahan W.B., Kellerman J. & Telfer T. (2003) Survival, dispersal and home-range establishment of reintroduced captive-bred puaiohi Myadestes palmeri. Biological Conservation, 111, 1-9
- Tweed E.J., Foster J.T., Woodworth B.L., Monahan W.B., Kellerman J.L. & Lieberman A. (2006) Breeding biology and success of a reintroduced population of the critically endangered puaiohi (Myadestes palmeri). The Auk, 123, 753-763
- Heath S.R., Kershner E.L., Cooper D.M., Lynn S., Turner J.M., Warnock N., Farabaugh S., Brock K. & Gracelon D.K. (2008) Rodent control and food supplementation increase productivity of endangered San Clemente Loggerhead Shrikes (Lanius ludovicianus mearnsi). Biological Conservation, 141, 2506-2515
- Cristinacce A., Handschuh M., Switzer R.A., Cole R.E., Tatayah R.V.V. & Jones C.G. (2009) The release and establishment of Mauritius fodies Foudia rubra on Ile aux Aigrettes, Mauritius. Conservation Evidence, 6, 1-5