Species and Community Profiles - San Francisco Estuary ...

Habitat Use and Behavior
California least terns forage by hovering over shallow to
deep waters and diving or, less often, dipping onto the
surface of the water to catch prey. Least terns also make
short skimming approaches onto pools of water left on
mudflats during low tide to catch trapped prey items.
Although California least terns have been known to consume
a wide variety of fish species, they appear partial
to northern anchovy (Engraulis mordax), and silversides
(Atherinidae sp.) (Atwood and Kelly 1984, Collins 1985).
To a much lesser extent there is evidence that least terns
may take small invertebrates such as the water borne
larvae of drone files, Eristalis tenax (Laura Collins, pers.
comm.; Leora Feeney, pers. obs.).
For nesting, least terns require tracts of open sand
or fine gravel substrate with sparse vegetation. Loss of
natural habitat has caused these birds to become opportunistic,
using areas such as newly filled or graded lands
and airports for nesting. Nesting areas must be located
near open water, usually along coastal beaches and estuaries,
and they must host adequate numbers of small
elongate fishes to sustain adults and growing young.
Conservation and Management
Human development of least tern habitat, highway access
to the coast, and summertime beach recreation have
caused the destruction of breeding sites and resulted in
least tern breeding failures. Although recovery efforts
have brought about increased least tern numbers in
California, some problems continue to challenge these
efforts. It appears that for colonies to have guaranteed
successes, they require intense management policies to
protect nest sites, including regular monitoring of breeding
activities, adequate barriers or supervision to restrict
public access, persistent predator control, and vegetation
management. These measures can be costly and funds
are not always available for known breeding sites to be
properly protected.
Predator management has become more difficult
due to the recent introduction of red fox (Vulpes vulpes)
on California’s coast. Feral cats and the establishment
of cat feeding stations in the State have added to least
tern reproductive failures (Edwards 1919, Caffrey
1995b, Feeney 1996). Public support for feral animals
has created additional problems with predator management
programs. In recent years, there has been concern
over reduced fish availability at some sites, which may
be related to “ El Niño” weather patterns or other phenomena
(Caffrey 1995a.).
To assure the future of a healthy least tern population
in the San Francisco Bay Area, adequate habitat
must be set aside and properly managed to support nesting
and post-breeding foraging. These protected areas
should be established at several locations around the Bay
to allow for alternative safe sites during potential localized
habitat crises.
References
Allen, A. 1933. The season: San Francisco Region. Bird-
Lore 35:336-337.
______. 1934. The season: San Francisco Region. Bird-
Lore 36:316.
Anderson, W. 1970. The California least tern breeding
in Alameda and San Mateo counties. Calif. Fish
and Game 56: 136-137.
Atwood, J.L. and P.R. Kelly. 1984. Fish dropped on
breeding colonies as indicators of least tern food
habits. Wilson Bull. 96(1): 34-47.
Barrantes, G. and A. Pereira. 1992. Abundancia y
fluctuaciones de aves limicolas (Charadriiformes)
en una playa fangs de Chomps, Costa Rica. Rev.
Biol. Trop. 40(3): 303-307.
Bender, K. 1973. California least tern census and nesting
survey, 1974. Ca. Dept. Fish and Game, Nongame
Wildl. Investigation W-54-R, Progress Rept. 47pp.
Brodkorb, P. 1940. New birds from southern Mexico.
Auk 57: 542-549.
Burleigh, T.D. and G. Lowery Jr. 1942. An inland race
of Sterna albifrons. Occ. Papers of the Mus. of Zool.
Louisiana State Univ. 10: 173-177.
Caffrey, C. 1995a. California least tern preliminary nesting
records. 1995 Season. Unpubl. report. Ca.
Dept. Fish and Game.
________. 1995b. Characteristics of California least tern
nesting sites associated with breeding success or
failure with special reference to the site at the Naval
Air Station, Alameda. Unpubl. report to Naval
Facilities Engr. Command, Western Div., San
Bruno, Calif.
California Department of Fish and Game (CDFG).
1981. Unpublished table of California least tern
colony estimated pair/fledgling numbers from 1978
through 1981.
Chandik, T. and A. Baldridge. 1967. Nesting season,
middle Pacific Coast Region. Audubon Field Notes
21(5): 600-603.
Chase, T. and R.O. Paxton. 1965. Middle Pacific Coast
Region. Aud. Field Notes 19:574-576.
Collins, L.D. 1985. California least tern nesting season
at the Alameda Naval Air Station, 1985. Appendix
2: Dropped fish collected at the least tern
colony, Alameda Naval Air Station, Calif. Rept.
prepared for the Ca. Dept. Fish and Game, and
the United States Navy.
________. 1988. The California least tern nesting season
at the PG&E Power Plant, Pittsburg, Calif. 1988.
Unpubl. Rept. to Ca. Dept. Fish and Game. 66pp.
Craig, A.M. 1971. Survey of California least tern nesting
sites. Ca. Dept. Fish and Game Project W-54-
Chapter 7 — Other Birds of the Baylands Ecosystem 361
Other Birds