The Latest on Least Terns

By Plover and Tern Program Director Ben Pearl

​California Least Terns were once abundant along the Pacific Coast from Monterey Bay to the tip of Baja California. But once people began to flock to the California coast in the late 1800’s, their population began a steep decline.As colonial ground nesting seabirds, Least Terns are highly vulnerable for a number of reasons. Their primary breeding habitat, coastal sand dunes, have been largely developed, and those that have not often host large numbers of beachgoers. Least Terns are very sensitive to human disturbance, and thus few suitable breeding sites remain. Where they do still breed, they face high levels of egg and chick predation, especially from predators such as corvids, gulls, ...

... skunks and foxes that are well adapted to humans. Furthermore, the small fish that they feed on, including anchovies and sardines, among others, have seen their own populations affected by the overfishing. All of these impacts culminated in their being listed as Federal and State Endangered in 1970 and 1971. Since that time, intensive conservation efforts have resulted in the population rebounding from a low of 600 pairs in 1973 to an estimate of 4100-5600 pairs in 2017.

Although Least Terns did not historically nest in the San Francisco Bay, wide scale changes to the bay ecosystem created suitable habitat for them. Currently, there are five breeding colonies spread across the bay, including one that SFBBO monitors at Eden Landing Ecological Reserve. The colony was established in 2017, initially attracted to the reserve by a large oyster shell enhancement that was spread for Snowy Plovers. Although Least Terns were successful breeders in 2017, since then they have experienced poor breeding success due to predation. 

​Beginning this coming breeding season, SFBBO plans to help improve their breeding success by enhancing their nesting habitat. On March 7, 2020, we will hold a volunteer event to remove vegetation and predator perches and spread two types of chick shelters, ceramic roofing tiles and wooden A-frames constructed by Dean and Nathan McCully. Both types of chick shelters are widely used at Least Tern breeding sites and have been shown to greatly reduce predation on chicks. To ensure that Least Terns take advantage of this enhanced habitat, SFBBO will deploy Least Tern decoys and an audio playback system to attract them to the site. SFBBO plans to more than double the amount of time spent monitoring the colony by our biologists and our volunteer monitoring program, for which we are currently accepting new volunteers.

If you are interested in attending the Least Tern Habitat Enhancement Event or becoming a Least Tern Volunteer monitor, contact me at [email protected] for more information.    

Ben Pearl, MS, is the plover and tern program director at SFBBO. Ben grew up in San Luis Obispo, where he attained an early love for nature exploring the nearby tide pools and oak forests. He completed his B.S. in Ecology and Evolutionary Biology at U.C. Santa Cruz, and first came to SFBBO while beginning his Masters at San Jose State University. For his Master’s thesis, he examined how various factors affect plover foraging habitat selection during the winter in the South San Francisco Bay. His favorite part of field work is seeing plover and tern chicks hatch and grow to become adults, especially when they are banded and he can keep track of them over the years. He also enjoys sharing his knowledge of these special birds through guided bird walks and public presentations.