Type: report

Article abstract: In 1993, a minimum of 2,400 pairs of the endangered California least tern (Sterna antillarum browni) nested at 35 sites along the coast of California, and produced approximately 1848-2009 fledglings. This 14% increase over 1992 breeding population size continues the trend since 1987 of continued growth of the population, and is directly attributable to the efforts of people working on behalf of recovery of the species. The statewide total of 2,400 pairs is the highest number recorded since systematic monitoring began in 1973, and represents a four-fold increase over the estimated 600 pairs of that year. Predation on tern eggs, chicks, fledglings, and adults, and abandonment of eggs and chicks as a function of predation pressure, were the major causes of breeding failure in 1993. Monitors at 15 of 22 sites with low fledgling production (<0.9 fledglings/pair) attributed the lack of success to predation. The adverse effects of predation were manifested at all stages of breeding. Many types of human-related disturbance also constrained fledgling production in 1993. Breeding success and failure were strikingly localized; successful (>0.9 fledglings/pair) and unsuccessful sites were distributed throughout the State. Seven sites were particularly successful at fledging high numbers of tern chicks: NAS Alameda, Venice Beach, Seal Beach, Mission Bay/Mariner's Point, and Delta Beach North combined produced approximately 82% of the total fledglings produced statewide.