The largest breeding colony of the endangered California Least Tern (Sterna antillarum browni) north of San Luis Obispo County occurs at Alameda Point, San Francisco Bay, California. Data on population size, reproductive success, and food habits were collected since the colony’s inception in 1976, while more specific data on breeding chronology (dates of active nesting and nest initiation dates), clutch size, hatching success, and size of prey dropped at the breeding site were collected from 2000-2004. The number of breeding pairs increased by 10.4% per year, making this colony the largest colony in San Francisco Bay and representing 6% of the state population. Breeding success varied but is generally superior to the state’s combined productivity numbers. Of the 32 Least Tern sites monitored in 2004, only five other colonies had higher breeding success than Alameda Point in that year. Despite the colony’s success, both hatching and breeding success have declined since the mid-1990s. Terns at this urban location forage in central and south San Francisco Bay, characterized by both marine and estuarine water. Dropped prey collected since 1981 indicate that silversides (family Atherinopsidae) were the most abundant prey in all years. Breeding success was positively correlated with the proportion of Northern Anchovy (Engraulis mordax, family Engraulididae), the second most common prey collected, suggesting that this high-energy fish may be beneficial to these terns. Potential factors limiting growth of this crucial northern Least Tern colony include predation and human disturbance, an inadequate protected nesting area, and environmental variability and its effects on prey abundance.
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Vol. 30 • No. 3