Effects of spatial distribution of corvids on nest predation risk

The Common Raven (Corvus corax) and the American Crow (Corvus brachyrhnchos) are both subsidized by anthropogenic resources. Two patterns of subsidized predation have been identified, spillover predation and hyperpredation, and spillover predation is likely to occur when predators use human-provided resources that are adjacent to natural habitats. In order to employ appropriate conservation strategies, it is important to understand the different predation patterns as these corvids do affect many threatened and/or endangered species. In this study crow and raven populations throughout the San Diego Zoological Society's Wild Animal Park were examined. Corvids are abundant there, often found in and around animal enclosures, where animal food and waste is present as well as where human provided resources are found. Corvid abundance declined with increasing distance to animal enclosures, but abundance was not affected by distance to public access areas. In contrast, attacks on artificial nests baited with both real and clay eggs did not significantly decrease with distance from either enclosures or public access areas. Furthermore, the number of ravens in the area did not help determine how long it would take for a nest to be attacked. It did, however, take longer for the ravens to find and attack nests that were far from both animal enclosures and public access areas, which suggests that spillover predation is occurring in the undeveloped habitat. Because spillover predation is occurring, it is possible that the ravens in the areas that are supported by anthropogenic resources could inhibit the growth of a prey population in the backcountry areas of the park. Key Words; spatial distribution, corvids, predation risk, crows, ravens, subsidized predators