Type: report

Article abstract: Approximately 4,017 pairs of California least terns (Sterna antillarum browni) nested at 38 sites along the coast of California in 1997, as reported by least tern monitors. This represents a 19%increase from 1996 pair estimates, and a 55% increase from 1995 pair estimates, more than compensating for the 7% decrease between 1994 and 1995. Recruitment cannot entirely account for this increase in least tern pairs, as reported fledgling production for both 1994 and 1995 was low. Immigration from other least tern populations has been suggested, and improved survival on wintering grounds may be a factor, but supporting data for these hypotheses are lacking. Overestimates of pair numbers and/or underestimates of fledglings by monitors may also partially explain this apparent increase. Reported pair and fledgling values are always imprecise estimates that are not scientifically derived; moreover, consistent methods for obtaining them, as recommended in annual monitoring packets, are not being used at all nesting sites. Reproductive success and adult survival in 1997 was affected by a number of predators, particularly at San Diego County sites. However, fledgling estimates (3,140 to 3,322) for 1997 were 55% to 64% higher than 1996 and 200% higher than 1995 estimates. The statewide fledglings-per-pair value (0.78 to 0.83) was also higher than for the previous three years. San Diego County supported over 57% of 1997 statewide pairs and produced over 58% of statewide fledglings at 20 nesting sites; the Santa Margarita River nesting sites alone supported over 18% of statewide pairs. More than 50%of the statewide breeding population was concentrated in six nesting sites (NAS Alameda, Venice Beach, Huntington Beach, Santa Margarita River North Beach, Mariner's Point, and Delta Beach North); these sites also produced over 64% of the State's fledglings. Santa Margarita River North Beach and NAS Alameda had the highest (over 1.2) fledglings-per-pair values. The Tijuana River nesting sites reported the lowest fledglings-per-pair value (0.01 - 0.02) in the State due to unprecedented burrowing owl predation on adults, a host of other predators in the site vicinity, and human disturbance and intrusion into the nesting site. No evidence of local prey shortages was reported by monitors for any nesting site in 1997.