At-sea congregation surveys to assess the status of Scripps's Murrelets Synthliboramphus scrippsi at islands off western Baja California, México in 2002-2008

In 2002−2008, we conducted spotlight surveys and at-sea captures to determine the distribution and to estimate the population size of Synthliboramphus murrelets at nine islands off the Pacific coast of Baja California (BC), México. Scripps's Murrelets S. scrippsi (SCMU) were detected in nocturnal at-sea congregations near six islands: Islas Coronado (IC), Todos Santos (TS), San Martín (SM), San Jerónimo (SJ), San Benito (SB), and Cedros (CD). Nest searches confirmed breeding at all islands except SM and CD, where breeding was presumed based on congregation attendance. Historically, SCMU were known or suspected to breed at all of these islands except CD, but knowledge of population size and trends prior to 1999 was limited to qualitative estimates and speculation. In 2002−2008, we estimated the total SCMU breeding population in the region to be 1686−4428 pairs, including 1117−2933 at IC, 262−688 at TS, 19−49 at SM, 24−64 at SJ, 231−607 at SB, and 33−87 at CD. SCMU populations have likely increased at IC, TS, SM, and SB since the eradication of cats in the late 1990s, but the small colony on SJ may be limited by competition for nest sites by a large and growing Cassin's Auklet Ptychoramphus aleuticus colony. Recent trends are unknown for the remnant SCMU population at CD, where breeding is restricted to isolated refuges safe from terrestrial predators. Although these surveys were conducted 11−17 years ago, these data provide the most recent population estimates available for these islands and offer a reliable modern baseline for measuring future population trends.