Effect of age at release on the post-release survival of head-started Spoon-billed Sandpipers

Head-starting, in which eggs are taken from the nests of wild birds, incubated artificially and the chicks reared in captivity before release to the wild, has been conducted since 2012 on the sub-Arctic breeding grounds of the Critically Endangered Spoon-billed Sandpiper Calidris pygmaea in an attempt to increase the production of fledged juveniles. To assess the effect of age at release on subsequent survival, we used resightings of individually marked head-started juveniles after they had moved away from the natal release site. We assessed differences in post-release survival of birds released at the same site on the same date in relation to differences in the age at which they were released (18–30 days). There was an unexpected significant tendency for survival to decrease with increasing age at release, both for survival from release to the period after departure from the release area and also to one year old. This effect of age at release is likely to be causal because multiple logistic regression analyses indicated that potential confounding effects of year of release and hatching date did not have statistically significant effects when included in models along with age at release. Possible explanations for the effect of age at release include increased stress or risk of disease or injury of individuals kept captive for longer periods. Our results suggest that releasing all juveniles at the typical age at fledging of wild birds (ca. 17 days) would improve post-release survival substantially. However, further experimental or correlative studies are required to identify the precise mechanism underlying the effect of age at release, and may reveal other procedural changes to potentially counteract it.