Kerry L Holcomb; U.S. Fish and Wildlife Service; kerry_holcomb@fws.gov; Peter S. Coates, Brian G. Prochazka, Timothy Shields, William I. Boarman

Decades of anthropogenic development and associated subsidies within the western U.S. has contributed to common raven (Corvus corax, raven) populations that exceed pre-subsidy ecoregion-specific carrying capacities. Consequently, ravens are implicated in declines of species of conservation concern. The juvenile life-stage (0–10 yr) of the California endangered and federally threatened Mojave desert tortoise (Gopherus agassizii, tortoise) has been shown to be exceptionally susceptible to raven predation. Using 274 variable-radius point counts, a database of 724 nesting raven territories, and 77 tortoise decoy trials, we evaluated the viability of tortoise-raven relationships under variable measures of contact. Specifically, juvenile tortoise decoy “survival” was modeled as a function of raven density and distance to the nearest raven nest. Annual survival was derived by adjusting decoy exposure to reflect natural activity patterns. Our model predicted that tortoise populations exposed to raven densities >0.89 raven km^2, at distances <1.72 km from a raven nest exhibit unstable inter-generational population structure, as excess mortality outpaced natural survival and reproduction. These results demonstrate that estimates of raven density, distance to nearest previously active raven nest, and decoy “survival” rates can inform development of a tortoise-raven viable conflict threshold. These findings are preliminary, and are provided for timely science communication and are subject to change.

Raven Management for Conservation Outcomes  Zoom Presentation