Breeding Of Craveri'S Murrelet Synthliboramphus Craveri At Four Islands Off West-Central Baja California, Mexico

Breeding by Craveri's Murrelet (CRMU) has been documented on many islands in the Gulf of California, Mexico. but has never been convincingly demonstrated on islands off the west coast of Baja California (BC). In 2007-2008. we confirmed CRMU breeding at Islas San Roque (n =1 nest), Asuncion (n = 2), Cedros (n = 2), and San Martin (n = 1) off west-central BC, using genetic analysis of eggshell membranes to confirm species identity. Spotlight surveys of Synthliboramphus murrelets attending at-sea congregations recorded 9-14 murrelets (n = 1 survey) at San Roque. 24-25 at Asuncion (n = 1), 168 at Cedros (n = 1), and 69 (+/- 15 SD; n = 2) at San Martin. All murrelets captured in at-sea congregations at San Roque (n = 6) were CRMU, but two species (82% CRMU. 18% Scripps's Murrelet S. scrippsi [SCMU]) were captured at Cedros (n = 11), and three species (63 % CRMU, 25% SCMU, 13% Guadalupe Murrelet S. hypoleucus) were captured at San Martin (n = 8). Applying a spotlight survey correction factor that quantified the relationship between the number of murrelets attending at-sea congregations and the number of nests on the adjacent shoreline, we tentatively estimated 14-22 CRMU pairs (95% C.I. = 10-40 pairs) at San Roque, 38-40 (95% C.I. = 26-72) at Asuncion, 221 (95% C.I. = 152-399) at Cedros and 69 (95% C.I. = 47-124) at San Martin. To our knowledge, these are the first population estimates for any CRMU colony. Confirmation of CRMU breeding at these four islands increased our understanding of Synthliboramphus murrelet distribution in the region. lending credence to presumed historical breeding by CRMU at Islas Natividad and San Benitos (where breeding has since been confirmed). More spotlight surveys, at-sea captures, and nest searches are needed to better determine the presence, size, and species composition of the murrelet populations on either side of BC.