The aim of this study was to analyze the variability observed in the food and feeding biology of the Common Tern (Sterna hirundo) in relation with environmental factors operating in nonbreeding season in Argentina at different spatial and temporal scales. Fish constituted more than 60% of the diet of the Common Tern in terms of biomass in all years, while insects never exceed 40%. A total of 25 fish species and nine insect species (Orders Coleoptera, Lepidoptera, Odonata and Hemiptera) were found in the diet. In four of six years, terns preyed more on marine fish (68%) while in 2001-02 terns foraged mostly on freshwater species (60%). Estuarine fish species were eaten in all years (>10%). Freshwater prey were found only in seasons with high local rainfall. Local wind-speed had a significant effect on the daily proportion of insects and fish prey found in the diet, with more insects (hence less fish) consumed on days with high winds. La Plata River overflow seasons were correlated with negatives values of the Southern Oscillation Index (SOI). We found less estuarine fish (e.g., Whitemouth Croacker, Pejerrey Silverside) after a month with a low SOI value. A positive trend was also observed between La Plata river outflow and more marine fish in the Common Tern diet. The observed environmental variability in winter quarters and the fluctuation in the use of different food sources may affect the individual quality of terns and not only the survival of individuals during the nonbreeding season but also the timing of migration or breeding performance of adults during the following reproductive season.
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Vol. 28 • No. 4