Post & D’Amico (2024)

Maggie Lee Post, Julie Thayer (Farallon Institute), and Lidia D’amico (National Park Service, Golden Gate National Recreation Area) continued the long-term monitoring of breeding seabird colonies on Alcatraz Island in San Francisco Bay. Brandt’s Cormorants (Urile penicillatus) nested in the highest numbers (n=4825) ever observed in the time series. Though heavy rain storms in early May flooded around one quarter of nests on the island, many pairs were able to re-lay and ultimately reproductive success was above the long-term mean (1995–2022). The frequency of anthropogenic disturbance to the cormorant colony increased over recent years; most disturbances were caused by recreational fisherman in small boats and frequent flyovers by helicopters and planes. The Western Gull (Larus occidentalis) breeding population remained stable (n=1065), and their productivity was above the long-term mean (1999–2022). The California Gull (L. californicus) colony was displaced by the growing Brandt’s Cormorant colony in 2022 and no pairs established nests on the island in 2023. Pelagic Cormorants (U. pelagicus) have not nested on the island since 2013. Two Black Oystercatcher (Haematopus bachmani) pairs nested on Alcatraz in 2023, though neither fledged chicks. Pigeon Guillemots (Cepphus columba) continued to nest on Alcatraz, with 62 nest sites confirmed with fish deliveries.