Reinforcement of an endangered goose population: the effect of age and interspecific fostering on survival of released birds

The release of captive‐bred or translocated individuals is a strategy used worldwide to support threatened animal populations. By capture–recapture analysis, we examined the survival of released individuals when reinforcing a critically endangered Lesser White‐fronted Goose population. Our analysis includes data from 646 birds, released 1984–2017, including two different age classes, divided in two distinct periods when different techniques were used. Fledglings were released with foster parents of Barnacle Geese in the first period and by a soft release without support of foster parenting in the second period. Yearlings were released by soft release in both periods. We find that use of foster parents enhances survival rates, but these differences were detectable only in the first year of life. Fledglings supported by foster parents showed significantly higher survival compared to yearlings released by soft release, but this difference was not clear when soft release was applied to both groups. Resighting data suggest that most losses occurred during the acclimation period following the release. Foster parenting may enhance survival rates due to social learning enabling the transfer of crucial behaviors (e.g. feeding, anti‐predator, and migration) to released individuals. However, these conservation benefits need to be balanced against costs and potential inherent risks related to foster parenting, including the imprinting of undesired behaviors in released individuals, such as hybridization. Based on our results, we advise conservationists to carefully consider foster parenting as one method to improve survival probability, especially if capacity to produce individuals to be released is a limiting factor.