The Fate of Released captive-reared Grey Partridges Perdix perdix: Implications for Reintroduction Programmes

We investigated the potential for released captive-reared grey partridges Perdix perdix to restock regions from which the species has disappeared. Birds were released at two sites in Scotland (not concurrently) from 1997 to 2003 and monitored via spring and autumn counts, night-time surveys and radio-telemetry. Some wild female partridges were caught and radio-tagged each spring for comparison with reared females. The survival rate of 520 captive-reared birds released in autumn until the following spring was poor (around 10% overall) and was not significantly different for radio-tagged and visibly-marked birds. Carrying a radiotag did not alter the body condition of the birds. The breeding-season survival rate of released hens averaged 30% and was not significantly different to that of 44% for wild hens. However, the power to detect significant differences was low due to the small number of survivors. The major cause of mortality throughout was predation (82 and 55% of losses at the two sites), with red foxes Vulpes vulpes and raptors being the most significant predators. Reared partridges at the study site with legal predator control had higher survival rates than those at the other site, but this was not true for wild hens as raptor predation compensated for declining mammalian predation rates (19% of deaths were due to raptors at the site without predator control, 56% at the site with predator control). We suggest that the vulnerability to predation of reared birds was most likely due to inappropriate antipredator behaviour or an increased risk of predation near to release pens. Of the partridges that survived long enough to breed, three times more wild hens reached incubation than reared hens. No reared hens raised chicks in their first breeding season whilst the only hen that survived long enough to breed in her second year raised 14 young. Captive-reared grey partridges could not be used to increase the species' range unless 1) the number surviving their first year increased, 2) the higher breeding rate of two-year-old females suggested here was ubiquitous, and unless 3) appropriate management was in place first. Methods for improving the success of releases are discussed.