Ocean-influenced estuarine habitat buffers high interannual variation in seabird reproductive success

Seabirds in more variable habitats generally live longer and more readily forgo or reduce breeding investments in poor resource seasons to maximize their overall lifetime fitness. Their breeding success is dependent on factors including diet, prey availability, and proximity to foraging habitat. Furthermore, seabird colonies in upwelling ecosystems are subject to interannual variation in oceanic conditions that drive bottom-up processes. Adjacent estuarine ecosystems, while less affected by upwelling, are also influenced by freshwater input and may experience less interannual variation in seabird prey resources. Here, we compare the breeding ecology and diet of pigeon guillemots Cepphus columba from an estuarine colony (Alcatraz Island, California) and an isolated offshore colony (Southeast Farallon Island, California) from 2015 to 2017 to understand how habitat location and surrounding environment differentially influence diet and reproduction. Breeding phenology in pigeon guillemots was similar between colonies, but reproductive success was higher and more stable at the estuarine site than at the offshore colony, where productivity was explained primarily by ocean conditions. Interannual and estuarine/offshore variability in chick diet composition was partially explained by upwelling and the influence of freshwater inputs. Variation in offshore pigeon guillemot productivity was related to the prey species composition. With increasingly variable conditions offshore in the California Current, the availability of alternative estuarine and nearshore breeding sites inshore may become increasingly important for the regional pigeon guillemot population and other seabirds capable of exploiting nearshore prey resources.