With Ocean Foraging Conditions All Over the Map, Alcatraz Offers Breeding Seabirds an Alternative

By Science Communication Specialist Jessica Weinberg McClosky, San Francisco Bay Area Network Science Communication Team

Pigeon guillemot pair on Alcatraz, carrying a sculpin. A new study finds that pigeon guillemot chick diet on Alcatraz varied somewhat less than on Southeast Farallon Island, suggesting the San Francisco Bay estuary buffered seabirds against the negative impacts of the powerful 2015 marine heatwave.

NPS / Laura Young

June 2022 - Seabirds often nest on remote, offshore islands. Roughly 30 miles off the central California coast, the Farallon Islands are a perfect example. They’re home to hundreds of thousands of breeding seabirds of a dozen different species. The surrounding California Current can provide great offshore foraging conditions thanks to upwelling of cold, nutrient rich waters from the deep sea. But ocean conditions are becoming more unstable (remember The Blob?) with more frequent impacts to seabird prey, and ultimately, breeding success. Meanwhile, estuaries like San Francisco Bay, home to Alcatraz Island’s seabird colony, may offer more stability. For example, past research found that over a five-year stretch, Alcatraz’s Brandt’s cormorants and western gulls had higher breeding success than their Farallons’ counterparts. Now, a study led by National Park Service Natural Resource Specialist Victoria (Tori) Seher lends further support to the idea that nearshore environments offer breeding seabirds a valuable habitat alternative.

Seher’s research focuses on one of Alcatraz’s least studied seabirds—the pigeon guillemot. Picture a jet black seabird with white wing patches and red feet, stocky yet sleek, and a bit larger than your average rock pigeon. The Alcatraz pigeon guillemot colony, at 50 breeding pairs in 2016, is the largest in San Francisco Bay, but much smaller than the Southeast Farallon Island (SEFI) colony Seher’s team also studied. The colonies were also different in a way that required a range of monitoring techniques. On SEFI, nests were easy to see and access. The researchers were able to weigh and band chicks on SEFI, and visually observe parents delivering prey to their chicks. But on Alcatraz, many of the nests were in inaccessible cliff cavities. Luckily, Seher could check the status of others visually and employ video surveillance of some nests to track prey deliveries.

Seher and her coauthors found that, between 2015 and 2017, pigeon guillemot pairs on Alcatraz had higher, more stable levels of success raising their chicks. Chick diet on Alcatraz also varied somewhat less, suggesting the San Francisco Bay estuary buffered seabirds against the negative impacts of the powerful 2015 marine heatwave. They caution that other factors, like shorter foraging distances in estuaries could also be at work, and that growing nearshore colonies could begin to attract more predators or experience more disturbances, causing any benefits to vanish. Success at nearshore breeding sites is also unlikely to substitute for the benefits of incredible boom years offshore. Still, Seher’s study suggests that, as offshore conditions become more variable, estuary sites like Alcatraz may become more important than ever to the success of pigeon guillemot and other seabird populations at a regional scale.

For more information

• Seher VL, Holzman BA, Hines E, Bradley RW, Warzybok P, Becker BH. (2022). Ocean-influenced estuarine habitat buffers high interannual variation in seabird reproductive success. Mar Ecol Prog Ser 689:155-167. https://doi.org/10.3354/meps14028
• Contact Natural Resource Specialist Tori Seher

---

See more from the Bay Area Nature & Science Blog