Nocturnal Spotlight Surveys for Monitoring Scripps's Murrelets in At-Sea Congregations at Anacapa Island, California

Abstract

Concealed nest sites, mostly inaccessible breeding habitats, and nocturnal colony visitation have long hindered studies of Scripps's Murrelets (Synthliboramphus scrippsi); but conspicuous and seasonally predictable at-sea congregations adjacent to nesting areas provide a valuable index for assessing murrelet colony size and population trends at all 12 breeding islands. We developed a boat-based nocturnal spotlight survey for counting murrelets in at-sea congregations at Anacapa Island, California. A high-intensity spotlight was used to count murrelets during standard surveys conducted on 2 parallel transects (1.9 km each) located 200 m (“inshore”) and 500 m (“offshore”) from shore. We conducted 130 standard surveys over 58 nights in 2001–2006. Difficulties estimating densities with strip transects and distance sampling led to use of raw counts as the best index for assessing population trends. Standard counts averaged 136 (SD 94) murrelets per survey (range 0–470). Strong correlations were noted among counts within nights, but consistent trends were not evident. We also noted a strong correlation between maximum counts on consecutive survey nights but much more variation in counts over the breeding season (annual CV range 27%–92%). Annual maximum counts occurred from 42 days before to 32 days after mean egg-laying dates. Round-island counts ranged from 29 to 564 murrelets during 12 surveys conducted along a 19.2-km transect circumnavigating Anacapa Island. Strong correlations between inshore and offshore standard counts and between standard and round-island counts indicated that major local shifts in distribution did not occur; instead, varying proportions of the murrelet population returned to the Anacapa Island congregation each night. Increases in standard counts from 2001 to 2006 and a positive correlation between standard counts and the annual number of clutches in monitored plots suggested an increase in the murrelet population after eradication of black rats (Rattus rattus) in 2002, but limited posteradication survey data were not sufficient to detect significant popu lation trends. Intensive surveys (minimum of 10 nights each year) over at least 3 consecutive years per decade are recommended to track the progress of the Scripps's Murrelet population and to validate rates of population growth observed in small nest plots on Anacapa Island.