Reproductive success of Common Ravens influences nest predation rates of their prey: implications for egg-oiling techniques

摘要

Abundance and distribution of Common Ravens (Corvus corax) have increased across western North America in recent decades, leading to conservation concern for sensitive prey species. Multiple management options have been proposed to reduce raven numbers or reduce the impacts of predation by ravens on sensitive species. We carried out a longitudinal study to evaluate the effects of experimentally manipulating the reproductive success of territorial ravens on the reproductive performance of their prey on Alcatraz Island across 6 years. Specifically, we employed remote video-monitoring on nests of predator and prey to document the effects of oiling raven eggs, a technique to prevent eggs from hatching, on raven reproduction, and to estimate impacts of raven hatching success on their predation of Black-crowned Night-Heron (Nycticorax nycticorax) nests. We used Bayesian multinomial logistic exposure models to estimate night-heron nest and chick survival and account for variation in nest failure across different life stages (i.e., egg versus nestling). We found daily predation rates on night-heron nests (median: 1.71%, 85% CI: 1.15–2.44%) and chicks (median: 0.57%, 85% CI: 0.27–1.03%) were the highest during years when ravens nested successfully. During years that raven eggs were oiled, daily predation rates on night-heron nests (0.98%, 85% CI: 0.55–1.57%) and chicks (0.00%, 85% CI: 0.0–0.00%) decreased, and we observed the lowest daily predation rates for nests (0.40%, 85% CI: 0.17–0.76%) during years when raven nesting was naturally absent on the island. The greatest probability of survival of night-heron eggs and chicks occurred during years when ravens were present but not nesting on Alcatraz. Our results suggest that manipulation of raven breeding success can influence predation and reproductive performance of their prey. These results also indicate that egg-oiling can be a viable localized method to reduce raven recruitment and possibly aid in the conservation of sensitive avian species.