Egg depredation by Common Ravens Corvus corax negatively affects Pelagic Cormorant Phalacrocorax pelagicus reproduction in central California

Abstract

Many corvid (family Corvidae) populations have increased worldwide, and their predation sometimes threatens populations of other species. We studied the effects of depredation by Common Ravens Corvus corax on nesting Pelagic Cormorants Phalacrocorax pelagicus at Año Nuevo State Park, California. We monitored a Pelagic Cormorant subcolony located on the Año Nuevo mainland with an automated camera during April to August 2014, and monitored all Pelagic Cormorant nests on the mainland and at Año Nuevo Island (located ~1 km from the mainland) during weekly checks. We also monitored breeding population size and reproduction of Pelagic Cormorants at Año Nuevo State Park from 1999 to 2014. During the 2014 nesting season, ravens visited the camera-monitored Pelagic Cormorant subcolony on the mainland 165 times, averaging 1.6 (standard deviation [SD] 2.5) colony visits/d (n = 101 d), interacted with 100% of nests (n = 13), and removed at least 3.3 (SD 2.2) eggs from each nest. We observed no raven interactions with Pelagic Cormorant nests on the island in 2014. Pelagic Cormorant productivity in 2014 was five times greater on the island (2.45 [standard error 0.2]; n = 29 nests) than on the mainland (0.48 [SE 0.2]; n = 27 nests). Likewise, nests on the island had significantly greater hatching and fledging success rates than those on the mainland in 2014. We conclude that low breeding success on the mainland in 2014 was caused by egg depredation by ravens, and that a single pair of breeding ravens was responsible. Comparison of 2014 results with those from previous years suggests that extreme mismatches of hatching success between adjacent Pelagic Cormorant colonies may signal localized egg depredation. These results demonstrate the importance of regular monitoring of coastal seabird populations to better understand impacts of raven depredation.