The Rarest Alcid: Status And History Of The Guadalupe Murrelet Synthliboramphus Hypoleucus At Isla Guadalupe, Mexico (1892-2007)

The Guadalupe Murrelet (GUMU) Synthliboramphus hypoleucus is the rarest alcid in the world (< 5 000 pairs), with regular breeding documented only at Islas Guadalupe (hereafter, "Guadalupe") and San Benito off the west-central coast of Baja California, Mexico. GUMU were discovered at Guadalupe in 1892, but by then feral cats Felis catus had already devastated the population and limited most nesting to predator-free islets just offshore. Despite its status as the only major GUMU breeding island, the population at Guadalupe was never adequately surveyed during the 20th century. In March-April 2007, we used survey techniques developed specifically for Synthliboramphus murrelets to assess the status and distribution of GUMU at Guadalupe. We counted 1 511 GUMU during spotlight surveys in waters around the island. GUMU were abundant near the islets, but they were also relatively numerous near suitable breeding habitats off the north and south coasts of Guadalupe proper. Searches on four islets yielded 93 nests, most at Islotes Zapato (66; 71%) and Morro Prieto (21; 23%), where scores of abandoned eggs indicated intense competition for nests. We found seven nests on Guadalupe proper, the first proof of breeding there since 1950, but we also found 68 carcasses that confirmed significant predation by cats and raptors. A spotlight survey correction factor yielded 2 418 breeding pairs (95% confidence interval = 1 662-4 367) at Guadalupe and its islets, which accounted for similar to 90% of the world breeding population (1 822-4 789 pairs). About 1150-1 750 pairs were estimated on Islotes Zapato and Morro Prieto in 1968, but comparable data were lacking to assess historical or recent changes in population size. Basic studies of GUMU biology have only recently been initiated and should be a continuing priority. Recent conservation actions have benefited the native biota of Guadalupe, but further measures, such as localized rodent control and robust biosecurity on the islets, should be considered to mitigate serious threats to the vulnerable GUMU population and other seabirds.