Species and Community Profiles - San Francisco Estuary ...

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land to South Carolina. They breed along the Gulf Coast
from Texas to Florida (AOU 1983). Caspian terns winter
from North Carolina south to Venezuela (AOU
1983).
On the Pacific Coast, Caspian terns breed from
British Columbia, Canada south to islands off the coast
to Sinaloa, Mexico. They winter from coastal central
California to northern Colombia (AOU 1983).
The first breeding accounts in the San Francisco
Bay Area are from the South Bay, where the majority of
Caspian terns nested prior to 1990. Since 1990, the
majority of birds nest at colonies in the Central Bay and
North Bay. Former colonies in the South Bay exist at
Baumberg, Turk Island, Bair Island, and Drawbridge/
Mowry. Current colonies exist at Pond A7 near Alviso
(104 pairs) and Coyote Hills (30 pairs). Single pairs have
recently nested among Forster’s terns at Ravenswood
Open Space Preserve, and Hayward Shoreline.
The only North Bay breeding colony exists at
Knight Island at the Napa River Marsh. This colony was
first detected in 1989 (R. Leong, pers. comm.). In 1977,
a small colony was detected at the Can Club, Island #2
(Gill 1977; R. Leong, pers. comm.).
In the Central Bay, colonies are found at Brooks
Island and at the Alameda Naval Air Station (NAS).
Population Trends – The largest continental
population of Caspian terns is found in North America,
with a minimum of 35,000 pairs nesting throughout
North America in the 1980s and 1990s (Cuthbert and
Wires 1999). The North American population has
steadily increased since at least the 1960s (Cuthbert and
Wires 1999).
Prior to 1970, the largest reported breeding colony
of Caspian terns on the Pacific Coast was in San Diego,
California (Gill and Mewaldt 1983). In the early 1980s,
the largest colony was at Gray’s Harbor in Washington,
where there were 2,157 pairs in 1981 (Gill and Mewaldt
1983). This colony was abandoned in the late 1980s, and
relocated at nearby Rice Island, Oregon. Currently, the
Rice Island colony is believed to be the largest breeding
colony, with 8,000+ pairs (Cuthbert and Wires 1999).
Other coastal colonies exist or have existed at Gray’s
Harbor and Willapa Bay, Washington; Columbia River,
Oregon; Humboldt Bay, Elkhorn Slough National Estuarine
Research Reserve, Salinas River Mouth, Bolsa
Chica Ecological Preserve, Newport Back Bay Ecological
Preserve, Salton Sea, and South San Diego Bay,
California (Gill and Mewaldt 1983; Cuthbert and
Wires 1999); and Laguna Figuroa (Palacios and Alfaro
1992), Scammon’s Lagoon (Bancroft 1927), and Laguna
San Ignacio (Danemann 1992), Mexico. Overall,
the Pacific Coast population has shown an increase
from at least 5,661 pairs estimated by Gill and
Mewaldt (1983) to at least 12,263 pairs estimated by
Cuthbert and Wires (1999). Most of this increase is
accounted for in the Gray’s Harbor/Rice Island colo-
356 Baylands Ecosystem Species and Community Profiles
nies where, roughly 2,300 pairs were estimated in the
early 1980s (Gill and Mewaldt 1983), and over 8,000
pairs were estimated in 1996-97 (Cuthbert and Wires
1999).
In 1922, Caspian terns were first documented nesting
in San Francisco Bay on a levee near the Dumbarton
Bridge in the Coyote Hills salt ponds. This small
colony of about seven nests increased over the next two
decades, with a total of 287 nests in 1931 (DeGroot
1931), and 378 nests in 1943 (Miller 1943). This colony
numbered approximately 299 pairs in 1966; however,
because of levee maintenance between 1968-69, the
colony was abandoned.
Several new colonies formed and disappeared in
other regions of the Bay. A colony established itself in
1968 near the town of Drawbridge, adjacent to Mowry
Slough, in Alameda County, which contained over 100
nests; 200 nests were counted in 1971 (Gill 1977). In
1968, colonies were also established at Baumberg and
Turk Island. The Turk Island colony rose to 540 nests
in 1985, but was abandoned in 1986, after levee maintenance
in 1985 (SFBBO 1988). In 1971, a colony was
discovered with 304 nests on a salt dike on outer Bair
Island. This colony grew to between 500-600 nests by
1975 (Gill 1977) and 800-850 nests by 1981 (Rigney
and Rigney 1981).
In the ensuing years, the activity and success at the
outer Bair Island colony was erratic. After decreasing to
200 nests in 1983 and being abandoned in 1984 and
1985, the colony was reestablished and included 1,700
adults (850 pairs) by 1988. The colony was again abandoned
in 1989 and then re-colonized in 1992 and 1993,
but no young were produced. In recent years, both the
Turk Island and outer Bair Island colonies have been deserted.
South Bay populations were estimated at 1,000-
1,200 individuals in 1971 (Gill 1973) and 2,350 individuals
in 1981 (Rigney and Rigney 1981). From 1984
to 1988 South Bay populations ranged from 1,120 to
2,111 adults (Woodin 1988), but in 1990 only 100 pairs
were reported (Woodin in Harvey et al. 1992). A Bay
Area-wide estimation of 2,818 individuals at five colonies
in 1989-90 was made by Carter et al. (1990). In
1997, an estimated 1,400 to 1,500 pairs nested in the
Bay Area, although only 136 pairs nested in the South
Bay.
Nesting colonies have been growing in the Central
Bay at Brooks Island and Alameda Naval Air Station
(NAS), concurrent with the colony abandonments in the
South Bay. The colony at Brooks Island began in the
early 1980s, grew to 400 adults (200 nesting pairs) in
1990 (Carter et al. 1990), and had reached 500 nesting
pairs in 1997. Six hundred pairs of Caspian terns were
recorded at the Alameda NAS colony in the early 1990s
(Harvey et al. 1992), and approximately 350 pairs nested
there in 1977.