The Nest Predator Assemblage for Songbirds in Mono Lake Basin Riparian Habitats

Because nest predation strongly limits avian fitness, ornithologists identify nest predators to inform ecological research and conservation. During 2002-2008, we used both video-monitoring of natural nests and direct observations of predation to identify nest predators of open-cup nesting riparian songbirds along tributaries of Mono Lake, California. Video cameras at 50 nests of 3 songbird species and direct observations of additional nests confirmed 10 distinct nest-predator species and suggested one additional species. Video-monitored nests experienced reduced predation rates, and video observations mainly captured nestling predation, even though predation rates were higher for non-video-monitored nests during the egg period. These findings suggest cameras may have missed some predators. By supplementing video records with field observations, we reduced the likelihood of excluding species from our predator list that substantially impact fitness due to camera-related biases. No single predator species emerged as predominantly important for shaping avian fitness. Nevertheless, we frequently observed garter snakes (Thamnophis sp.) depredating nestlings and Brown-headed Cowbirds (Molothrus ater) depredating eggs. Corvids and rodents were also identified as potentially frequent groups of nest predators. Video cameras recorded partial predation by 5 predator types, as well as several cases in which nests were visited by multiple predators. Finally, 6 of 7 predators that were video-recorded depredating artificial nests were species also documented depredating natural nests. These observations verify the relevance of artificial-nest data for further study of these predators.