SPOTLIGHT SURVEYS FOR GRASSLAND OWLS ON SAN CLEMENTE ISLAND, CALIFORNIA

Abstract

According to Breeding Bird Survey data, grassland birds are among the most imperiled species in North America. Within this group, grassland owls show steep population declines across the United States. Despite these declines, questions still remain regarding the seasonal and geographic distribution of grassland owls. On San Clemente Island (SCI), California, grassland owls are known to occur, but nothing is known about their distribution or abundance. To increase our understanding of owl populations on SCI, we used night-time spotlighting to survey for grassland owls from October 2001 to October 2002. We recorded 733 detections of three species of owls: Barn Owl (Tyto alba), Burrowing Owl (Athene cunicularia), and Short-eared Owl (Asio flammeus). Barn (8.3 ± 0.8 owls/hr) and Burrowing owls (2.2 ± 0.7 owls/hr) were the most frequently detected species, whereas Short-eared Owls were rarely detected (0.2 ± 0.1 owls/hr). We detected owls during all nighttime hours surveyed and detected Barn Owls in every month of the study. We detected Burrowing Owls only from October to March and Short-eared Owls from December to April, suggesting that they are winter visitors. Despite the bias of increased detectability using roadside surveys, spotlighting from a vehicle enabled us to efficiently cover a large proportion of the island (compared to walking surveys) and survey multiple grassland species using one survey technique.