Human-created Habitats

Meredith Elliott and Sara Acosta (PRBO Conservation Science); Mark J. Rauzon (Laney College)

Status: Bird populations are stable.

Several human-created habitats are used by birds: levees, bridges, and buildings, to name a few. Data on the bird use of all these habitats are not available. In this section we discuss two key places, Alcatraz Island and some of the Bay’s bridges, where bird monitoring data exist.
Alcatraz, once a barren sandstone rock originally inhabited by seabirds, faced a long period of human settlement. In the last 20 years, the island has once again begun to attract seabirds that use its human-created structures as home.
Over 100 feet above the water the I-beams and other support structures under the roadways of the Richmond–San Rafael Bridge and the San Francisco–Oakland Bay Bridge serve as nesting platforms for Double-crested Cormorants.

Brandt’s Cormorants (blue) and Western Gulls (red) – Alcatraz Island reproductive success.

Double-crested Cormorants – Number of nesting pairs on Bay bridges.

Above left, Richmond-San Rafael Bridge.
Above right, Western Gull at nest site, Alcatraz Island.

Seabirds are long-lived birds; thus their populations can withstand occasional years of poor reproduction or reduced reproductive effort. Because of this, evaluating the status of a nesting colony is done by looking at the reproductive success or breeding population size. The figures at left present over two decades of data on reproductive effort for cormorants and gulls.

Western Gull and Brandt's Cormorant: Stable. In 20 years of monitoring the nesting success of these long-lived seabirds, reproductive success has remained largely constant until 2009 and 2010. The complete nesting failure in 2009 and 2010 was likely due to a low anchovy population throughout the Central California coast region.

Double-crested Cormorant: Stable, though their reproductive success has varied over the last 26 years. In 2009 and 2010, these cormorants showed a sharp decline, but they appear to be recovering. The low number of nesting pairs in 2009 was likely due to a low anchovy population throughout the Central California coast region.


Primary threat: Human disturbance, including maintenance activities, tourism, and boating, can cause seabirds to abandon the nesting colony.

Lack of food due to steep declines in common prey species, as evidenced by the 2009 region-wide anchovy crash, can cause seabirds not to breed.

Losing nesting sites on human-created structures. On Alcatraz, if nesting areas are opened for public tourism during the spring and summer, nesting habitat will be lost.

Contaminants accumulation in adult birds, in high concentrations, can affect reproduction and chick survival. Cormorants are fish-eating birds and are at risk of accumulating contaminants (e.g., mercury, lead) from San Francisco Bay.

Climate change effects such as extreme high temperatures result in heat stress in nesting birds (nausea, dizziness, seizures, death) and nest abandonment, as witnessed in 2008 on Alcatraz Island.

Double-crested Cormorant


Planning, Management, Restoration

Stop tourism, maintenance, and construction activities and manage tourism to reduce bird disturbance during the months of February–July. If not possible, maintenance and construction personnel should work with biologists on ways to limit disturbance.

Create new habitat on bridges and piers when possible. Explore using methods of social attraction to draw birds to newly built 'cormorant condos' (artificial nesting structures on the new San Francisco-Oakland Bay Bridge).

Re-install historic buoys at Alcatraz and work with the Bay Conservation Development Commission to implement seasonal closures to create a boat-free buffer zone during the seabird nesting season.

Educate tourists on Alcatraz about the sensitivity of nesting seabirds. National Park Service programs should continue to increase tourist awareness of nesting seabirds on Alcatraz, especially with regard to closed areas during the nesting season.


Assess contaminants through studies of nesting birds. Failed- to-hatch eggs have been collected opportunistically from the bridge colonies, but eggs should be collected and analyzed for contaminants on an annual basis. Relating cormorant contaminants with prey contaminants can help identify which fish species carry the most contaminants in San Francisco Bay. Further research on lethal levels of these contaminants in Double-crested Cormorants should be considered.

Prey studies are needed. A better understanding of Double-crested Cormorant diet is needed, since food affects the survival of this population.

Conduct complete annual monitoring of all known Double-crested Cormorant breeding sites, especially the South Bay power towers.

Success Story • Seabirds on Alcatraz Island

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