Increase of tropical seabirds ( Sula ) in the California Current Ecosystem with warmer ocean conditions

Climate change is impacting marine ecosystems in the California Current Ecosystem (CCE) through physical changes (e.g., an increase in the frequency and intensity of marine heatwaves) that manifest biologically at all trophic levels. We investigated range expansion into the CCE and correlations with environment for a group of tropical/sub-tropical seabirds. We assessed changes in the abundance (2002-2022) of five species from the genus, Sula (Cocos, Blue-footed, Red-footed, Masked, and Nazca Boobies), using a novel compilation of four data sources (at-sea surveys, n=82 observations, records from Southeast Farallon Island, n=600, the California Rare Birds Committee, n=593, and eBird -a citizen science mobile application, n= 20,529), and looked for relationships with the environment, including broad temporal and spatial scale dynamics (El Niño Southern Oscillation), local conditions where the bird was reported, and potential source conditions from Baja California Sur and the Gulf of California. All five species increased in abundance, and all, with the exception of Blue-footed, exhibited a northward range expansion by as much as 6.8 degrees latitude and increased range area of between 235-1013% within the CCE. There was increased abundance during warmer conditions (El Niño and warm SST) for Cocos, Red-footed, Masked, and Nazca Boobies, and all species increased by 692-3015% after the extreme marine heatwaves that began in late 2013. Our results document a tropical shift in seabirds of the CCE, which may present future challenges to resident species. As marine heatwaves are projected to increase in frequency and intensity, in addition to long-term warming, we hypothesize that these species will continue to expand their range northward in the CCE.