Release captive-bred individuals into the wild to restore or augment wild populations of gamebirds
Overall effectiveness category Unknown effectiveness (limited evidence)
Number of studies: 5
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 review of a 1978-89 reintroduction programme for cheer pheasants Catreus wallichii in northern Pakistan (Garson et al. 1992) found that post-release survival was low between 1978 and 1985 (see â€˜Use â€˜anti-predator trainingâ€™ to improve survival after releaseâ€™ for details), but that 10-15% of 305 birds released in 1986 and 1988 survived at least one year after release. Breeding was recorded in 1987 (one pair nested, nine eggs laid, seven hatched but all chicks died within six weeks) and 1989 (three females nested, with eight chicks surviving to at least eight weeks and leaving the release pen). However, a survey in 1990 suggested that the release area (a national park) was becoming too overgrown to support cheer pheasants, and that rotational burning (similar to traditional agriculture) may be necessary to maintain the population in the area. Releases were conducted at a medium elevation site (700m) in 1978-81, and two higher elevation sites (approximately 1,000 m) from 1983 and 1988 respectively. Birds were released into open-topped release pens, with 54 birds released between 1978 and 1981; 279 released in 1983-6 and 305 in 1988-9.Study and other actions tested
A before-and-after study in western Germany between 1980 and 1992 (Spittler 1994) reports that a western capercaillie Tetrao urogallus reintroduction programme succeeded in establishing a population of 30-40 individuals by spring 1992, with breeding recorded in 1986. Before the releases, the species had died out in the region probably by 1974. A total of 393 birds (226 males, 167 females) were released during the study period, with 200 in the last four years. A large number of released birds were predated, but some released males lived for five to seven years following release. Before release, chicks were hatched in incubators, raised in â€˜post-hatchâ€™ cages until four months old and then moved to large outdoor pens for a month before release. The authors argue that the wild population size is too small to be self-sustaining in the long-term.Study and other actions tested
A controlled, replicated study (1991-1996) in mixed arable land in central Finland found that, due mainly to poor survival and low reproductive success, releasing hand-reared female grey partridges Perdix perdix contributed little to boosting the local wild population (Putaala & Hissa 1998). Hand-reared females had lower survival during the breeding period than wild females (19% vs. 69%) and wild partridges produced more fledglings than released ones (2.09/female vs. 0.05/female). There was no significant difference in spring dispersal (3.1 km wild; 2.3 km hand-reared), nesting chronology, clutch size (wild average 20.5 eggs vs. hand-reared 19.3 eggs), or nest predation (main cause of mortality in both sets of birds) between wild and hand-reared birds.Study and other actions tested
A review of a reintroduction programme in two prairie sites in Texas, USA, in 1996-7 (Lockwood et al. 2005) found that two-week survival rates of 119 released, captive-bred and hand-reared Attwaterâ€™s prairie chickens Tympanuchus cupido attwateri (an endangered subspecies of the greater prairie chicken) were 51-82%. The date of release (July-October), release habitat (prairie or soybean â€˜food plotâ€™) or type of radio-transmitter used to track birds did not affect six-month survival rates, whereas time spent in cages prior to release did (discussed in â€˜Use holding pens at release sitesâ€™). Movements and range-sizes were similar for released and wild birds but there was no known recruitment into the population from released birds. Mortality was mainly from predation, whilst known nesting failures appeared to be due to invasive red fire ants Solenopsis invicta.Study and other actions tested
A replicated, controlled study in two arable farmland areas in Angus, Scotland, found that, due to poor survival and low reproductive success, releasing commercially reared grey partridges Perdix perdix did not contribute to bolstering declining wild populations (Parish & Sotherton 2007). Studies were conducted at one site in autumn 1997 to summer 2001 and a second in autumn 2001 to summer 2004. Eight-week old commercially-reared partridges were placed in release pens in September each year and released 2-3 weeks later. Released birds (520) were monitored by spring and autumn counts, night-time surveys and radio-telemetry. Some wild female partridges were radio-tagged for comparison. Survival of captive-reared birds from autumn to the following spring was low (averaging 10%). Breeding-season survival of released females averaged 30% and for wild females 44%. The major cause of mortality was predation (69% of losses). Of the few reared birds that survived to breed, none fledged chicks in their first breeding season. Only one released female survived to breed in her second year, but this individual raised 14 young.Study and other actions tested