Abundance and foraging behavior of pursuit-diving birds in Cattle Pass

Abstract

Marbled Murrelets (Brachyramphus marmoratus), Rhinoceros Auklets (Cerorhinca monocerata), and Pelagic Cormorants (Phalacrocorax pelagicus) share similar foraging styles, choice of prey, and habitat preference within the San Juan Channel. In this study we used shoreline surveys to compare and contrast the nearshore foraging behavior and abundance of these three pursuit-diving species in Cattle Pass, Washington. We sought to assess tidal and temporal influence on abundance and compare diving behaviors among species, specifically focusing on: dive and rest times, lateral dive movement, and foraging sociality. We found that MAMU abundance peaked in the morning, RHAU were more abundant in the late afternoon, and PECO abundance had a bimodal distribution. Separately, MAMU and PECO were more abundant during fast currents, while RHAU were most abundant nearshore during slack high tide, likely because they were foraging solitarily for their chicks. We observed that the alcid species tended to travel laterally while diving more often than PECO and that among our three target species, foraging sociality was most important in MAMU.