ABSTRACT
Understanding sex differences in foraging behaviour has important implications for breeding biology and population ecology. This requires the evaluation of segregation throughout the annual cycle, including the non-breeding period. We studied the diet and foraging strategies of the Common Tern (Sterna hirundo) at an intraspecific level by sampling birds during two consecutive non-breeding periods in Punta Rasa, Buenos Aires Province, Argentina. Blood samples were taken for sex determination and carbon and nitrogen stable isotope analysis. No isotopic difference was observed between years, but higher isotopic values occurred during late non-breeding period. Carbon and nitrogen isotopic values were similar between sexes; however, differences were observed in standard ellipse analyses, considered to represent isotopic niches. Even though the Bayesian ellipse areas overlapped, females showed wider isotopic niches throughout the non-breeding period. This could be related to different feeding strategies, foraging areas or migratory stopover sites at either intraspecific or individual levels. Diet contributions of different prey categories estimated through mixing model analyses suggested a higher contribution of estuarine fish than of marine pelagic species. These results highlight the importance of stable isotope analyses as a complementary methodology to traditional diet studies, particularly during the non-breeding period.
Acknowledgements
We are grateful to Gabriel Castresana, Pablo O. Rojas, Juan Ordoñez, Melina Lunardelli and all other Organismo Provincial para el Desarrollo Sostenible park ranger staff for the logistic support in capturing birds. We thank Matilde Cavalli, Oscar Stellatelli for field support and Teresa Ravasi also for lab support. We are grateful to Daniel Alonso and people from CERC (Municipalidad del Partido de la Costa) for kindly providing lodging support in San Clemente del Tuyú. We appreciate the improvements in English usage made by Peter Lowther through the Association of Field Ornithologists’ programme of editorial assistance. Thanks also to three anonymous reviewers for their valuable input that greatly improved the manuscript.
Supplemental material
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