A specimen-based approach to reconstructing the late Neogene seabird communities of California

California has experienced major climate change from the early Miocene (~ 23 Ma) to the present. Fossil seabirds are useful for studying marine faunal responses to these changes because they are numerous in museum collections and easily identified to the family-level from fragmentary remains. Because extant seabirds respond quickly to environmental changes, we hypothesize that the fossil record of crown-group Miocene seabirds will reflect past environments. Previous studies of fossil seabird diversity through the Cenozoic of the North Pacific relied mainly on literature records to describe the appearance and disappearance of seabird species and correlated these patterns to geologic and climatic events. Our study uses an empirical, specimen-based approach to describe the seabird response to climate and tectonic change during ~ 12 million years of coastal California's geologic history (middle Miocene to early Pliocene). The foundation of our dataset is 242 fossil seabird specimens from Orange County. The strata from Orange County form the basis for delineating chronostratigraphic bins used for studying 285 additional seabird fossils from other parts of California, for a total of 527 seabird fossils examined. Our results show a relationship between pelagic seabirds and offshore facies as well as a clear increase of pan-alcid abundance and decline of sulid abundance through time. This pattern may be the result of taphonomic bias combined with the effect of shallower facies preserved through time. The increase in pan-alcid abundance is also coincident with global climatic and tectonic changes and with enhanced and stable nutrient upwelling. Upwelling through this time also helps to explain morphological changes in salmon and speciation rates in marine mammals. The specimen-based methodology used here can be applied to contemporaneous taxa, such as marine mammals, to quantitatively analyze diversity during the late Neogene and further explore the relationship between physical drivers and faunal change.