Type: report

Article abstract: Monitoring to document breeding success of California least terns (Sternula antillarum browni) continued in 2015, with observers at 41 nesting sites providing data. An estimated 4202-5295 California least tern breeding pairs established 5504 nests and produced 1514-1887 fledglings at 49 documented locations across California. The fledgling to breeding pair ratio was 0.29 to 0.45. Statewide, 9654 eggs were reported, with a Statewide clutch size of 1.7 eggs (St Dev = 0.26) for Type 1 sites where monitors walk within the colony. Numbers of nesting least terns were not uniformly distributed across all sites. Camp Pendleton, Naval Base Coronado, Huntington, Point Mugu, and Alameda Point each had over 300 minimum breeding pairs, which represented 64% of the state total, and produced the most fledglings, contributing 60% of the state’s production (Table 1). Sites with greater than 35 fledglings each (the five aforementioned sites plus Mariner's Point, Hayward, Batiquitos, Bolsa Chica, and Oceano Dunes) contributed 86% of the state’s production. The 2015 statewide non-predation chick mortality rate was 18%, similar to that in 2014 (20%). With the exceptions of Camp Pendleton and Naval Base Coronado, the larger nesting colonies experienced non-predation chick mortality rates less than the average, similar to that documented in 2014. The predators known to be responsible for the greatest number of depredated least terns in 2015 were coyotes (Canis latrans), common ravens (Corvus corax), corvid species, raptor species, peregrine falcons (Falco peregrinus), and American crows (Corvus brachyrhynchos). The monitoring effort of 2015 is scheduled to continue in 2016.