Data from: Seasonal and annual differences in the foraging ecology of two gull species breeding in sympatry and their use of fishery discards

Niche segregation between similar species will result from an avoidance of competition but also from environmental variability, including nowadays anthropogenic activities. Gulls are among the seabirds with greater behavioural plasticity, being highly opportunistic and feeding on a wide range of prey, mostly from anthropogenic origin. Here, we analysed blood and feather stable isotopes combined with pellet analysis to investigate niche partitioning between Audouin’s gull Larus audouinii and yellow-legged gull Larus michahellis breeding in sympatry at Deserta Island, southern Portugal, during 2014 and 2015. During the breeding season there was considerable overlap in the adults’ diet, as their stable isotope values of blood and primary feather (P1) did not differ, and their pellets were comprised mainly by marine fish species. However, Audouin’s gulls presented higher occurrences of pelagic fish, while yellow-legged gulls fed more on demersal fish, insects, and refuse. SIAR mixing models also estimated a higher proportion of demersal fish in the diet of yellow-legged gulls. We also found differences between the two gull species in chicks’ feathers, suggesting that Audouin’s gull adults selected prey with lower carbon isotope values to feed their young. Secondary feather (S8) of Audouin’s gull presented higher isotope values compared to yellow-legged gulls, indicating different foraging areas (δ13C) and/ or trophic levels (δ15N) between the two species in the non-breeding season. During both the all-year and non-breeding periods the yellow-legged gull showed a broader isotopic niche width than Audouin’s gull in 2013, and in 2014 the two gull species exhibited different isotopic niche spaces. Our study suggests that both gull species foraged in association with fisheries during the breeding season. In this sense, a discard ban implemented under the new European Union Common Fisheries Policy may lead to a food shortage, therefore future research should closely monitor the population dynamics of Audouin’s and yellow-legged gulls.

相似物种间的生态位分化（niche segregation），既可由种间竞争规避所驱动，也可源于环境异质性，包括当下的人类活动。鸥类是行为可塑性最强的海鸟类群之一，兼具高度的机会主义取食策略，食谱涵盖多样猎物类群，其中多数源自人为活动来源。本研究整合血液、羽毛稳定同位素（stable isotopes）分析与食团分析（pellet analysis），旨在探究葡萄牙南部德塞尔塔岛同域繁殖的奥杜邦鸥（Audouin’s gull, Larus audouinii）与黄腿鸥（yellow-legged gull, Larus michahellis）之间的生态位分隔（niche partitioning），采样时段为2014年与2015年。繁殖季期间，两种鸥的成鸟食谱存在显著重叠：二者血液与初级飞羽（primary feather, P1）的稳定同位素值无统计学差异，且其食团主要由海洋鱼类构成。然而，奥杜邦鸥的食团中远洋鱼类（pelagic fish）的出现频率更高，而黄腿鸥则更多取食底栖鱼类（demersal fish）、昆虫与生活垃圾。SIAR混合模型（SIAR mixing models）的估算结果同样显示，黄腿鸥的食谱中底栖鱼类占比更高。我们还发现两种鸥的雏鸟羽毛同位素特征存在差异，表明奥杜邦鸥成体会选择碳同位素值更低的猎物饲喂幼鸟。奥杜邦鸥的次级飞羽（secondary feather, S8）同位素值高于黄腿鸥，暗示二者在非繁殖季的觅食区域（以δ¹³C表征）和/或营养级（以δ¹⁵N表征）存在分化。无论全年还是非繁殖时段，2013年黄腿鸥的同位素生态位宽度（isotopic niche width）均大于奥杜邦鸥；2014年两种鸥的同位素生态位空间也存在显著差异。本研究表明，繁殖季两种鸥均与渔业活动存在觅食关联。据此，欧盟共同渔业政策（European Union Common Fisheries Policy）下推行的渔获丢弃禁令，或可能引发食物短缺，因此未来研究应密切监测奥杜邦鸥与黄腿鸥的种群动态。