Alcatraz, a Safe Harbor for Seabirds When the Ocean Turns Unreliable

By Jessica Weinberg McClosky, science communication specialist, San Francisco Bay Area Network, June 2022.

A pigeon guillemot pair on Alcatraz with a sculpin. NPS / Laura Young.

Offshore islands, unstable seas

Seabirds usually nest on remote offshore islands. The Farallon Islands, about 30 miles off central California, hold hundreds of thousands of breeding seabirds of a dozen species, fed by the California Current, where upwelling brings cold, nutrient-rich water up from the deep.

But ocean conditions are growing more unstable — remember The Blob? — hitting seabirds' prey and breeding. Estuaries like San Francisco Bay, home to Alcatraz's seabird colony, may be steadier. Earlier research found that over five years, Alcatraz's Brandt's cormorants and western gulls bred more successfully than their Farallon cousins.

The pigeon guillemot study

A study led by NPS natural resource specialist Victoria (Tori) Seher looked at one of Alcatraz's least-studied seabirds, the pigeon guillemot: jet black with white wing patches and red feet, stocky yet sleek, a bit bigger than a rock pigeon.

• Alcatraz colony: 50 breeding pairs in 2016, the largest in San Francisco Bay, but much smaller than Southeast Farallon Island's (SEFI).
• Methods: on SEFI, nests were easy to reach, so the team weighed and banded chicks and watched parents deliver food. On Alcatraz many nests were in inaccessible cliff cavities, so Seher checked others by eye and used video to track food deliveries.

The findings

• Steadier success: from 2015 to 2017, Alcatraz pairs raised chicks with higher, more stable success.
• Steadier diet: chick diets varied somewhat less on Alcatraz, suggesting the Bay buffered the birds against the powerful 2015 marine heatwave.
• Caveats: shorter foraging trips in estuaries may also help; growing nearshore colonies could attract predators or disturbance; and they cannot match the boom years offshore.

Still, as offshore conditions grow more variable, estuary sites like Alcatraz may matter more than ever for pigeon guillemots and other seabirds across the region.

Sources

Based on "With Ocean Foraging Conditions All Over the Map, Alcatraz Offers Breeding Seabirds an Alternative," by Jessica Weinberg McClosky, San Francisco Bay Area Nature & Science Blog, National Park Service; a work of the United States government in the public domain. The study: Seher, V. L., B. A. Holzman, E. Hines, R. W. Bradley, P. Warzybok and B. H. Becker, 2022, "Ocean-influenced estuarine habitat buffers high interannual variation in seabird reproductive success," Marine Ecology Progress Series 689: 155–167.