Type: report

Article abstract: Monitoring to document breeding success of California least terns (Sternula antillarum browni) continued in 2012, with observers at 41 nesting sites providing data. An estimated 4293-6421 California least tern breeding pairs established 6636 nests and produced 557-628 fledglings at 49 documented locations. The fledgling to breeding pair ratio was 0.09 to 0.15. Statewide, 11,036 eggs were reported, with a Site Mean clutch size of 1.66 eggs per nest (St Dev = 0.134) and a Statewide clutch size of 1.65 eggs (St Dev = 0.492) for Type 1 sites. Numbers of nesting least terns were not uniformly distributed across all sites. Naval Base Coronado, Point Mugu, Batiquitos Lagoon Ecological Reserve, Camp Pendleton, Huntington State Beach, and Alameda Point represented 74% of the breeding pairs. Fledgling numbers were also unevenly distributed as the two sites with at least 90 fledglings each (Hayward and Huntington State Beach) contributed 38% of the state’s production, and the five sites with greater than 35 fledglings each (Hayward, Huntington State Beach, Oceano Dunes, Seal Beach, and Lindbergh Field) contributed 59% of the state’s production. The 2012 chick mortality rate was 49%, continuing the upward trend observed in the previous four years. With the exception of Batiquitos Lagoon and Camp Pendleton, the larger nesting colonies experienced chick mortality rates less than the average, which is opposite that documented in 2011. The most commonly-reported possible, suspected, or documented predators were common ravens (Corvus corax), American crows (Corvus brachyrhynchos), peregrine falcon (Falco sparverius), great-blue herons (Ardea herodias), American kestrels (Falco sparverius), and red-tailed hawks (Buteo jamaicensis). The predators known to be responsible for the greatest number of depredated least terns in 2012 were American kestrel, coyote (Canis latrans), common raven, American crow, unknown avian species, unknown gull species, gull-billed tern (Gelochelidon nilotica), red-tailed hawk, peregrine falcon, and northern harrier (Circus cyaneus). The monitoring effort of 2012 is scheduled to continue in 2013.