Strong effects of radio‐tags, social group and release date on survival of reintroduced grey partridges

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AbstractAnimal reintroductions are important tools for conservation but often fail for reasons poorly understood. Biological and methodological factors can affect reintroduction success in complex manners. Investigating dispersal after release often relies on the use of biologging devices but individuals cope differently with these devices. They can detrimentally affect the performance of individuals depending on life‐history stage and may provide flawed conclusions, because inference drawn from tagged individuals cannot be readily generalized to non‐tagged individuals. Furthermore, environmental conditions during the release period or the expression of social behaviour such as the coordination of flocking behaviour can affect the outcome of reintroductions. In a reintroduction project involving 1385 grey partridges (Perdix perdix) released in 54 social groups (i.e. coveys), we investigated whether individual biweekly post‐release survival varied throughout different life‐history stages and was affected by radio‐tags of different weight (8 vs. 11 g), covey affiliation and release date. We conducted experiments in captivity to evaluate whether physiological stress, body condition and individual take‐off performance were affected by radio‐tags. During the first month after release survival was low in radio‐tagged and untagged birds. Throughout the second‐ and third‐month post‐release, negative effects of radio‐tags on survival were apparent. Four months after release, the survival of radio‐tagged and untagged birds converged. Consequently, the annual survival of untagged birds was more than five times higher than that of radio‐tagged birds. We did not find clear negative effects of radio‐tags on stress hormone levels but take‐off performance tended to be lower in tagged birds. Survival strongly decreased with later release date and differed between coveys. The use of radio‐tags can detrimentally affect individual survival and reintroduction success of grey partridges. The risk and benefits of radio‐tags should be balanced and releases in reintroductions should be conducted under the most favourable social and environmental conditions.