Type: report

Article abstract: Monitoring to document breeding success of California least terns (Sternula antillarum browni) continued in 2016, with observers at 42 nesting sites providing data. An estimated 3989-4661 California least tern breeding pairs established 4746 nests and produced 1612-2000 fledglings at 50 documented locations across California. The fledgling to breeding pair ratio was 0.35 to 0.50. Statewide, 7891 eggs were reported, with a Statewide clutch size of 1.37 eggs (St Dev = 0.64) 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, Batiquitos, Point Mugu, Huntington, and Alameda Point each had over 300 minimum breeding pairs, which represented 72% of the state total, and produced 63% of the state’s fledglings. Sites with greater than 35 fledglings each (the five aforementioned sites plus Hayward, LA Harbor, Huntingon, Bolsa Chica, and Oceano Dunes) contributed 88% of the state’s fledgling production. Least tern mortality due to non-predation factors was greater than mortality due to predation in 2016. Of non-predation egg mortality events, the highest cause of failure was attributed to abandonment prior to the expected hatching date and abandonment post-term. The 2016 statewide non-predation chick mortality rate was 29%, higher than that in 2015. The number of fledglings and adults that died in 2016 due to non-predation factors was similar to that in 2015. The predators known to be responsible for the greatest number of depredated least terns in 2016 were peregrine falcons, rats, common ravens, northern harriers, and coyotes. The monitoring effort of 2016 is scheduled to continue in 2017.