Post-Release Dispersal and Predation of Head-Started Juvenile Desert Tortoises (Gopherus Agassizii)Effect of Release Site Distance on Homing Behavior

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Head-start nurseries have been proposed as a possible means of promoting recovery of Desert Tortoise populations. However, when released near their long-term nursery pens, juvenile Desert Tortoises (Gopherus agassizii) have been shown to initially attempt to return to their natal pen, which could have consequences for social interactions, spread of disease, and predation risk. We released 16 juvenile tortoises 500 m away from their home pen to determine whether this distance would eliminate such site fidelity. We tracked tortoises for three months following release. We monitored location, habitat use, activity, and survival. Tortoises showed no tendency to return to the natal pen following release; most settled into one location within two weeks of release. Seven of 16 tortoises were killed over a six-week period, apparently by a single Common Raven (Corvus corax). Predation risk was significantly affected by size; only tortoises with masses < 125 g were taken. Head-starting of tortoises to a larger size could result in higher survival rates, and releases at least 500 m from the natal pen could promote more normal dispersal. However, site fidelity could also be a useful management tool if it is desirable for tortoises to remain near their release location. Behaviors other than dispersal may also be altered by long-term residence in nursery pens, and further studies are warranted.

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