Marine Ornithology

The Craveri's Murrelet Synthliboramphus craveri is an endangered seabird (family Alcidae) endemic to northwestern Mexico, where it breeds on rocky islands throughout the Gulf of California (GOC). Nesting records are scarce for most of these islands, and much of the available information relies on historical data collected more than 50 years ago. Beginning in 2009 through to 2023, we performed intermittent nest searches on five islands in the Midriff Islands of the GOC. Starting in 2018 on Alcatraz Island and starting in 2022 on San Pedro Mártir Island, we monitored nests to determine average clutch size and hatching success. We also performed nocturnal spotlight surveys around San Esteban Island and San Pedro Mártir to estimate breeding populations. On Alcatraz, we observed breeding behavior from December to April and chicks departing nests as early as January. During 2020-2023, average clutch size on Alcatraz was 1.6 eggs per active nest and average hatching success was 1.02 eggs, with eggs hatching in 68% of active nests. While we observed numerous murrelets in the waters surrounding some islands, we did not find much evidence of nesting on islands with invasive rodents. On San Pedro Mártir, where invasive rodents were exterminated in 2007, we estimated the breeding population to range from 38 pairs in 2022 to 99 in 2023. Future studies are needed to further quantify the breeding population and to better understand the specific impacts of both invasive rodents and variation in ocean climate upon murrelet nesting. Finally, it is important to examine the causes of variation in nesting success rates and phenology over the years to increase the effectiveness of conservation efforts.