California Least Tern

Meredith Elliott (PRBO Conservation Sceience); Susan Euing (U.S. Fish and Wildlife Service)

San Francisco Bay Status: Stable

The largest colony in San Francisco Bay is located at Alameda Point on the runway complex of the former Naval Air Station, Alameda. The 3.9 hectare breeding area is surrounded by a chain link fence.
Smaller colonies can be found at Napa–Sonoma Marshes Wildlife Area, Montezuma Wetlands, and Hayward Regional Shoreline.

Breeding pairs of Least Terns in the Alameda Point colony. While the colony has grown over the last 26 years, it appears to be stabilizing.

Average percent of each fish in the diet of Least Terns in the Alameda Point colony. Small fishes are the dominant prey item.

Since 1984, the number of California Least Tern pairs in the Alameda colony has increased by 9.7% per year, but the colony size appears to have stabilized in the last decade.

Dropped prey items have revealed the diet and foraging habits of Least Terns at the Alameda Point colony from 1981 to present. They show that small estuarine fishes are the dominant prey item. Since the 1990s, northern anchovy and surfperches have declined in the tern’s diet, while Clupeids (e.g. herring, sardine) have increased. Understanding prey items is important because diet is critical to tern reproductive success.

Threats

Primary threat: Avian predators including falcons, hawks, owls, crows, and ravens prey upon young and adult terns. Human activities, such as leaving food for predators and altering native habitat, result in higher than normal predator populations.

Development of the Naval Air Station looms, as the base has been decommissioned.

Encroaching vegetation is reducing the nesting area available to the birds. The California Least Tern needs bare ground for nesting and roosting. Encroachment of vegetation reduces the amount of nesting habitat.

Low flying aircraft over the nesting colony flush adult terns from their nests, leaving young and eggs vulnerable to predators and unfavorable weather conditions.

Sea level rise from climate change threatens to submerge the colony site, as it is built on reclaimed land close to sea level.

Loss of common prey species, such as the region-wide anchovy crash in 2009, can result in fewer feedings to chicks, nutritional deficiencies, and higher rates of chick death.

Contaminants can directly kill birds, but they also kill the prey items that birds depend upon for food. California Least Terns feed on fish and are at risk of accumulating contaminants (e.g., mercury and lead) found in San Francisco Bay. These contaminants, in great enough concentrations, can affect survival and breeding success.

Actions

Planning, Management, Restoration

Continue predator management, especially of Peregrine Falcons. The increase in local Peregrine Falcon attacks on the terns at Alameda Point is a growing concern. Authorization to permanently remove marauding Peregrine Falcons from Least Tern sites needs to be given to predator management personnel by the appropriate state agency.

Control vegetation by continuing to apply herbicide, remove weeds, and add gravel to the nesting substrate.

Reduce air traffic disturbance by expanding outreach to local airports and pilots regarding impacts to the endangered Least Tern.

Secure/identify adequate undeveloped space beyond the existing colony to allow for colony movement or persistence in the long term, given the uncertain future of the Navy’s presence.

Scientists

Study the diet and energy requirements of developing terns and evaluate the nutritional content of common prey species.

Monitor contaminant impacts to terns. Failed-to-hatch eggs should continue to be collected and analyzed for contaminants. Further research on lethal levels of these contaminants in Least Terns is needed.


Previous page: Western Snowy Plover
Next page: Northern Spotted Owl