Winter pre-conditioning of seabird phenology in the California Current

Climate change is predicted to affect the phasing and amplitude of upwelling in eastern boundary current marine ecosystems. Effects may be strongest during the spring or summer ‘upwelling season,’ but may also be influential during winter before the spring transition. We tested the hypothesis that wintertime environmental forcing ‘pre-conditions’ the ecosystem and affects the timing and success of breeding in 2 species of seabird, Cassin’s auklet Ptychoramphus aleuticus and common murre Uria aalge, reproducing in the central California Current. Time series of the mean and variance of egg-laying dates were correlated with regional winds and sea surface temperature, which were used as proxies for the forcing and oceanic response of coastal upwelling, respectively. Winter proxies of upwelling were the most significant determinants of seabird reproductive phenology, with anomalously strong upwelling in January to March leading to earlier dates of egg-laying in both species. We hypothesized that anomalous (early) winter upwelling, even of weak magnitude or short duration, could seed the region with sufficient nutrients to enhance productivity and prey availability, leading to a healthier adult breeding population and an earlier start to the breeding season. The magnitude of the previous year’s November winds were also positively correlated with the variance of egg-laying dates for the common murre, with upwelling-favorable winds leading to greater synchrony in egg-laying. We conclude that winter environmental conditions are important determinants of ecosystem dynamics in the California Current, and that seabird phenology is a particularly useful ecosystem indicator.