A first population assessment of black Oystercatcher Haematopus Bachmani in California

Abstract

Black Oystercatcher Haematopus bachmani is considered vulnerable to decline owing to small global population size, low reproductive success and complete dependence on rocky intertidal shorelines that are impacted by human use and rising sea levels. In response to poor baseline knowledge of the population of the species in California, during 2011 we undertook the first targeted survey measuring distribution and abundance. For the mainland, we used a standardized protocol developed specifically for detecting Black Oystercatchers during the early breeding season, when pair fidelity to breeding territories is highest and movement is lowest. For the Channel Islands, government biologists used a standardized seabird monitoring protocol adapted to detect Black Oystercatchers. For the Farallones, data are taken from the literature. On the mainland, 164 observers participated in the survey in 12 of the state's 15 coastal counties. Observers surveyed approximately 9% of the mainland California coast, equalling approximately 18% of the state's mainland suitable habitat, defined below. A total of 1 160 Black Oystercatchers were detected in this subset of habitat, more than the previous estimate for the entire state (<1 000 individuals). Average density of individuals in mainland surveyed areas was 3.14 birds/km; 135 nests were positively identified, and average nest density was 0.4 nests/km surveyed. On the Northern Channel Islands, approximately 20% the total coastline (66 km), and 20% of the suitable habitat (58 km) of San Miguel, Santa Rosa, Santa Cruz, Anacapa and Santa Barbara islands was surveyed. A total of 176 adult or sub-adult Black Oystercatchers were detected. Average density of individuals in surveyed areas was 2.7 individuals/km, comparable to mainland densities. In areas thoroughly surveyed, observers found densities of nesting territories comparable with those in Alaska and British Columbia, considered the core of the species' range. Based on observed densities in surveyed areas, and including estimates from the literature and more recent observations of a population of 60 at the Farallones, we conservatively estimate a total mainland and Farallones population between 3 971 and 5 213 and a Northern Channel Islands population between 779 and 854, for a total statewide population between 4 749 and 6 067. Our results indicate that California is a critical rather than peripheral part of the Black Oystercatcher range. This result, plus ongoing threats, emphasizes the need to monitor Black Oystercatcher population trends and to identify and protect the most important habitats for Black Oystercatchers in California. The Black Oystercatcher appears highly amenable to citizen science monitoring, particularly at smaller spatial scales, owing to its life history characteristics and charismatic appeal to the public.