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.

中文翻译：

---

越冬地单形海鸟普通燕鸥 (Sterna hirundo) 同位素组成的种内变异

摘要 了解觅食行为的性别差异对育种生物学和种群生态学具有重要意义。这需要在整个年度周期中对隔离进行评估，包括非繁殖期。我们通过在阿根廷布宜诺斯艾利斯省蓬塔拉萨的两个连续非繁殖期对鸟类进行采样，在种内水平上研究了普通燕鸥 (Sterna hirundo) 的饮食和觅食策略。采集血样用于性别确定和碳氮稳定同位素分析。年份之间没有观察到同位素差异，但在非繁殖晚期出现更高的同位素值。两性之间的碳和氮同位素值相似；然而，在标准椭圆分析中观察到差异，被认为代表同位素生态位。即使贝叶斯椭圆区域重叠，雌性在整个非繁殖期间也显示出更广泛的同位素生态位。这可能与种内或个体水平的不同饲养策略、觅食区或迁徙中途停留地点有关。通过混合模型分析估计的不同猎物类别的饮食贡献表明，河口鱼类的贡献高于海洋中上层鱼类。这些结果强调了稳定同位素分析作为传统饮食研究的补充方法的重要性，尤其是在非繁殖期。通过混合模型分析估计的不同猎物类别的饮食贡献表明，河口鱼类的贡献高于海洋中上层鱼类。这些结果强调了稳定同位素分析作为传统饮食研究的补充方法的重要性，尤其是在非繁殖期。通过混合模型分析估计的不同猎物类别的饮食贡献表明，河口鱼类的贡献高于海洋中上层鱼类。这些结果强调了稳定同位素分析作为传统饮食研究的补充方法的重要性，尤其是在非繁殖期。