Captive rearing conservation programs focus primarily on maximizing post-release survival. Survival increases with size in a variety of taxa, often leading to the use of enhanced size as a means to minimize post-release losses. Head-starting is a specific captive rearing approach used to accelerate growth in captivity prior to release in the wild. We explored the effect of size at release, among other potential factors, on post-release survival in head-started Mojave desert tortoises Gopherus agassizii . Juvenile tortoises were reared for different durations of captivity (2–7 y) and under varying husbandry protocols, resulting in a wide range of juvenile sizes (68–145 mm midline carapace length) at release. We released all animals ( n = 78) in the Mojave National Preserve, California, United States, on 25 September 2018. Release size and surface activity were the only significant predictors of fate during the first year post-release. Larger head-starts had higher predicted survival rates when compared to smaller individuals. This trend was also observed in animals of the same age but reared under different protocols, suggesting that accelerating the growth of head-started tortoises may increase efficiency of head-starting programs without decreasing post-release success. Excluding five missing animals, released head-starts had 82.2% survival in their first year post-release (September 2018–September 2019), with all mortalities resulting from predation. No animals with >90 mm midline carapace length were predated by ravens. Our findings suggest the utility of head-starting may be substantially improved by incorporating indoor rearing to accelerate growth. Target release size for head-started chelonians will vary among head-start programs based on release site conditions and project-specific constraints.

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