Thayer & Barnes (2025)

Author Information

Julie Thayer: Farallon Institute

Morgan Barnes: National Park Service

We have monitored Alcatraz seabird population size, breeding phenology, and productivity since the island was colonized by Brandt’s Cormorants in the early 1990s. Disturbance to wildlife is a concern because Alcatraz is the most heavily-visited tourist destination in Northern California. Island visitation returned to full capacity in 2022 following two years of reduced or no visitation during the COVID-19 pandemic in 2020–2021.

After a record number of Brandt’s Cormorants in 2023 (almost 10,000 birds, representing a significant proportion of the meta-population), the Alcatraz breeding population and productivity were significantly lower in 2024. Heavy and recurring late rains, warm oceanic conditions, and fewer foraging opportunities likely contributed to this drop in breeding cormorants on the island.  

The Western Gull population numbers remained stable, but 2024 productivity was low. However, Pigeon Guillemot numbers continue to increase, as have Black Oystercatcher populations, although oystercatcher productivity remains low. California Gulls and Pelagic Cormorants no longer breed on Alcatraz Island.

One adult Western Gull carcass in 2024 tested positive for Highly Pathogenic Avian Influenza, although all subsequent birds tested have been negative (including one Brandt’s Cormorant in early 2025).