Description
For seabirds in particular, linking varying resource abundance and distribution to reproduction can be challenging due to the difficulty of observing foraging directly as well as the accurately sampling highly mobile prey in a dynamic oceanic habitat. This lack of information on the relationship is particularly problematic Endangered Species Act listed species. Here we focus on the endangered California least tern (Sterna antillarum browni), a management reliant species, which has had variable and often low reproductive success in recent years. Using stable isotope analysis on salvaged eggs and long-term breeding datasets for colonies in San Diego County, CA, we found significant annual variation in resource availability. In years characterized by low measurements of _15N California least terns initiated nesting later in the season and generally had higher fledgling rates. In years where higher trophic level prey was abundant in the pre-breeding diet we found that nesting was initiated earlier in the season and fledgling rates were lower. In order to indentify underlying influences of these observed shifts in least tern diet and associated changes in reproductive patterns, we used remotely sensed oceanic data for temperature, chlorophyll concentration, and upwelling to characterize environmental conditions. We found a significant negative relationship between the diet signal of foraging females and upwelling in the Southern California Bight during the time period of egg development. Furthermore, reproductive output was driven by different factors dependent on stage of breeding. Greater proportions of females initiated nests within the first two weeks of the breeding season when the diet was dominated by lower trophic level prey while local sea surface temperature was a more important factor on other measures of reproduction such as clutch size and hatching and foraging success rates. Our research provides critical context for the variable California least tern reproductive output observed on nesting colonies in the past decade. The findings of this study highlight the need for a richer understanding of the interactions between local ocean conditions, diet and California least tern breeding output in light of the need to manage and recover this species.
