Synthesis of nest predation impacts of common ravens on sensitive avian species

Decades of mounting scientific evidence have revealed that common raven (Corvus corax; raven) population numbers have been increasing across nearly all regions of their geographic range in North America. Concomitantly, numerous native wildlife species have experienced elevated predation rates from ravens as populations have increased and expanded their range. Managers are concerned that increased raven predation of many threatened and endangered avian species in the U.S. and Canada during nesting periods may be hampering species recovery. We explored the literature to aggregate existing knowledge and evaluate the impacts of raven predation on nests and young of sensitive avian species. We used this information to develop a simple relative index for each species, the “Raven Impact Index” (RII). The RII incorporated the species demographic rates, abundance of ravens in relation to each sensitive species’ breeding range, and the degree of overlap between raven and sensitive prey distributions. We also developed a second relative descriptor describing our confidence in each RII, termed a “Impact Credibility Index (ICI).” The species ICI was based on the number of published studies and the type of evidence presented (e.g., circumstantial vs. direct). We found evidence of nest predation on 8 sensitive avian species and suspected nest predation on 1 additional species. All species shared aspects of nesting biology that suggested they would likely be susceptible to raven nest predation. The RII varied among prey species, with greater sage-grouse (Centrocercus urophasianus) having the highest relative impact values, followed by snowy plover (Charadrius nivosus nivosus), marbled murrelet (Brachyramphus marmoratus), and Gunnison sage-grouse (Centrocercus minimus). Our species RII is intended to inform management decisions regarding actions that mitigate the negative effects of raven predation of sensitive avian species. Although elevated nest predation may be of high conservation concern, it is important to recognize that all of the sensitive native prey species we established an RII for also face multiple conservation threats.