Scripps's Murrelet at San Miguel Island, California: Status of a Small Population at the Northwest Limit of the Breeding Range

. San Miguel Island (SMI) and its adjacent islets, Prince Island and Castle Rock, support a small population of Scripps's Murrelets (Synthliboramphus scrippsi) at the northwest limit of their breeding range. In 1994–1996, 2004, 2007, and 2015, we conducted at-sea surveys and nest searches to determine the breeding distribution and population size of the Scripps's Murrelet at the SMI group. During at-sea vocal surveys in 1994–1996, murrelet calls were heard at 4 stations around Prince Island (43–167 calls), 2 stations off Castle Rock (6 calls and 80 calls), and 4 stations off northeast SMI (5–123 calls), but none were detected at 8 stations off southern and northwestern SMI. At-sea spotlight surveys in 2004, 2007, and 2015 confirmed murrelet congregations off Prince Island and northeast SMI. Annual mean (±SD) spotlight counts on a standard transect off of northeast SMI and Prince Island were 9 ± 0 murrelets (n = 2) in 2004, 19 ± 4 (n = 3) in 2007, and 91 ± 15 (n = 2) in 2015; much higher spotlight counts at Prince Island accounted for the higher mean in 2015. We found murrelet nests in 4 of the 12 areas searched at SMI and Prince Island. In 2007 the first murrelet nests ever found on SMI proper were discovered at Harris Point, Bay Point, and Hoffman Point in isolated shoreline habitats that were inaccessible to endemic Island Foxes (Urocyon littoralis littoralis) but accessible to nonnative Black Rats (Rattus rattus). Using spotlight counts we estimated 100–263 breeding pairs at the SMI group in 2015, mainly at Prince Island (90%) where foxes and rats were absent. Spotlight surveys have not been conducted at Castle Rock, but <5 pairs were suspected. Introducing rats from SMI proper to the offshore islets is the most serious threat to murrelets at this remote island, although a large oil spill could also extirpate this population. Spotlight surveys and nest searches should be conducted at least every 5 years to confirm persistence of this vulnerable species at SMI and to detect major changes in population size.