Intraspecific variability in isotopic composition of a monomorphic seabird, the Common Tern (Sterna hirundo), at wintering grounds

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.

KEYWORDS:

• Stable isotopes
• isotopic niche
• non-breeding period
• feeding ecology
• terns
• sexual segregation

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.

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Additional information

Funding

This work was supported by the National Agency for Scientific and Technical Promotion (PICT 1401-2012); Universidad Nacional de Mar del Plata (UNMdP,EXA745/15 and 15/E975-EXA842/17); and CREOi. The Organismo Provincial para el Desarrollo Sostenible (OPDS) permitted sampling of Common Terns in Punta Rasa. R.M.J. was funded by the National Research Council of Argentina (CONICET) and E.M. was funded by the Mar del Plata National University (UNMdP).