Alcatraz's Record Waterbird Nesting Season, 2021

Waterbirds nested on Alcatraz Island long before people built on it, and in recent decades they have been reclaiming parts of it. Even so, the island's natural resources staff had never seen a season like 2021, which broke records by a wide margin.

Brandt's cormorants on Alcatraz. NPS / Lidia D'Amico.

Brandt's cormorants

These sleek black diving birds, with blue throat patches, began returning to Alcatraz in the early 1990s with only a handful of nests. In 2021 staff counted 3,301 breeding pairs, beating the previous record of 2,648 pairs, set the year before, by 653. How big the colonies were centuries ago is unknown, but it is the most breeding cormorants the island has held in modern times.

They also started early: the first egg turned up in a nest on March 3, about 4–6 weeks earlier than usual.

A Brandt's cormorant feeding one of its three chicks. NPS / Lidia D'Amico.

Why such a good year

• Plenty of food. Ocean conditions for northern anchovy, the cormorants' main prey, were excellent. With food so abundant, some pairs raised a second brood after successfully fledging their first.
• A quiet island. The COVID-19 closure of Alcatraz, which lasted into March, may have spared nesting birds from human disturbance.
• Years of care. Staff believe their decades of work to reduce disturbance have improved breeding conditions.

Great blue herons

The island's great blue herons nest in a single large eucalyptus above the Alcatraz rose garden. In 2021 this rookery held 12 active nests, the most yet, compared with 4–6 in earlier years, and chicks had begun to fledge by August.

Great blue heron chicks in the eucalyptus rookery. NPS / Lidia D'Amico.

Peregrine falcons

A pair of peregrine falcons nested on Alcatraz for the second time. The female, Lawrencium, is a banded bird from the Campanile Tower nest on the University of California, Berkeley campus. She and her mate raised three chicks, a boost for the Bay Area's peregrines.

One of the three young peregrines raised on Alcatraz in 2021. NPS / Lidia D'Amico.

Sources

Rewritten from the National Park Service article "Alcatraz Hosted Unprecedented Numbers of Waterbirds During 2021 Nesting Season" (August 2021; public domain), in hubnx's own words. The article's heron chick counts do not agree — 26 documented, of which 14 had fledged and 11 were still in the nest — so no count is given here.