The California Least Tern (“tern”) is federally listed as “endangered”. Efforts to recover this species have been successful, with a population increase from about 600 breeding pairs to well over 6000 pairs over the past 40 years. Unfortunately, there are still significant threats, due mainly to the finding that 70% of the breeding population nests at only 5 sites. Our study sites account for the highest densities of those nests. Threats to terns include: limitation and reduced quality of habitat, predation by native and non-native predators, human disturbance, and long-term factors such as food availability, disease and climate.