Recent Snowy Plover population increase arises from high immigration rate in coastal northern California

The Pacific coast population of the Snowy Plover Charadrius nivosus nivosus is listed as threatened under the U.S. Endangered Species Act, which requires demographic data to inform management directed at increasing the population. Accordingly, we summarized a 16-year dataset on population size and growth, return rates, and productivity of a color-marked breeding population in coastal northern California, one of six recovery units for the listed population. The geographically isolated population varied annually in size (19–74 breeding adults), with an early nine-year decline (λ = 0.92 ± 0.30) followed by a seven-year increase (λ = 1.22 ± 0.19). Overall, productivity averaged 0.85 ± 0.29 chicks fledged per male, which is well below that identified by viability analyses to maintain the population. Adult return rates, an index of survivorship, varied greatly among years (30–95%). Immigrants comprise 63 ± 5% of the population over the past 12 years when we have marked approximately 95% of breeding adults. We conclude that the northern California population is a demographic sink, and that management actions currently are insufficient to increase the population; recent growth stems from immigration.