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Description
The Gull-billed Tern (Gelochelidon nilotica) is a facultative intraguild predator of both California Least Tern (Sternula antillarum browni) and Western Snowy Plover (Charadrius nivosis nivosis) chicks in San Diego, California. While this predation has been documented, no information is available on the underlying dynamics that influence these biotic interactions. During 2010-2011, I implemented a movement and diet study of Gullbilled Terns to document inter- and intra-annual variation in adult movements and chick diet for terns breeding in San Diego, California. Using active and stationary radio telemetry, I documented both movements of adults throughout San Diego Bay and attendance of individual Gull-billed Terns at particular California Least Tern and Western Snowy Plover breeding areas. With the use of stable isotope analyses and chick provisioning observations, I characterized diet using a three isotopic tracer mixing model and corroborated the model results using the chick provisioning observations. Chapter 1 highlights Gull-billed Tern variation in movements and diet in response to both biotic and abiotic influences of food resource dynamics and the energetics demands of reproduction. These data suggest that Gull-billed Tern home ranges are driven by food resource dynamics and the demands of raising offspring and will vary both inter- and intraannually based on these influences. Chapter 2 details diet characterization of Gull-billed Terns at two life stages using complementary methods of stable isotope diet characterization and direct observation of chick provisioning. Results of the analyses reveal that mole crabs are a primary diet contribution for both adult females and chicks, while Least Tern and Snowy Plover chicks are not primary contributors to the Gull-billed Tern chick diet. Chapter 3 highlights the variability of Gull-billed Tern attendance at four Least Tern and Snowy Plover breeding areas in San Diego, California. Gull-billed Terns were found to have a high level of both individual and temporal variability in attendance. Attendance was highest during the day, with lowest attendance during 1400-2159. Night activity was documented which is a first for this species.
