Dietary Plasticity Mitigates Impacts to Reproduction for the Gull-Billed Tern Gelochelidon nilotica During Abnormally Warm Sea Surface Temperature Events in California, U.S.A.

Large scale oceanic processes can have profound consequences for marine and coastal food webs. Mortality and reproductive related impacts to seabirds have been documented for decades, and current research suggests that dietary flexibility may be a key component by which birds can mitigate environmental variation. Our motivation for this research was to better understand how a dietary generalist in the coastal environment responds to changes in prey food availability. The Gull-billed Tern Gelochelidon nilotica is a coastal nesting species that has an opportunistic generalist diet. We monitored both tern diet and density of a main prey resource to examine how responsive these terns are to annual variation in prey resources. Our results documented that the loss of a ubiquitous prey resource did not appear to influence tern annual reproductive success even though, in some years, Emerita analoga comprised greater than 70% of Gull-billed Tern diet. During breeding seasons with warmer than average sea surface temperatures, the Gull-billed Tern switched to a more terrestrial diet and focused aquatic foraging activities upon crustacean species that are more tolerant of warmer water temperatures. Dietary plasticity can be beneficial to mitigate variation in prey resource availability and impacts on reproductive success, and the ability to respond rapidly to changes in resources may play an important role in how coastal species can acclimate or adapt to annual changes in the prey base.