ABSTRACT Capsule: Great Black-backed Gulls Larus marinus breeding on Skokholm, UK, fed predominantly on seabirds, rabbits, refuse, and marine prey, with the majority of pairs being dietary generalists, but with some specialist pairs. Aims: To understand the significance of Great Black-backed Gulls as top predators on a small offshore island with internationally important numbers of breeding seabirds (Skokholm, UK) by quantifying their diet and to determine how this varies within the breeding season, to test for pair-level dietary specialization and to examine the consequences of dietary differences for reproductive performance. Methods: Regurgitated pellets were collected and analysed from 26 breeding pairs on Skokholm during 2017 and related to breeding success. Results: Analysis of 1035 pellets revealed that, overall, Great Black-backed Gulls fed on seabirds (48% – mostly Manx Shearwaters Puffinus puffinus), mammals (38% – mostly European Rabbits Oryctolagus cuniculus), anthropogenic waste (7%), and marine prey (7%). Diet varied among pairs with 18 (73%) generalist pairs and 7 (27%) specialist pairs (of which, 5 were bird specialists and 2 were mammal specialists). Diet also varied seasonally, but pair-level dietary diversity was repeatable through the breeding season. Dietary diversity did not covary with breeding success. Conclusion: Great Black-backed Gulls are top predators on Skokholm. Variation in diet among pairs emphasizes that not all individuals contribute equally in terms of predation. Understanding the incidence of this variation has important ecological implications, particularly where apex predators may exert a strong top-down influence.

中文翻译：

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鸟类顶级捕食者饮食的种群内变异：大黑背鸥 Larus marinus 的通才和专家觅食

摘要胶囊：大黑背海鸥 Larus marinus 在英国 Skokholm 繁殖，主要以海鸟、兔子、垃圾和海洋猎物为食，大多数对是饮食通才，但也有一些专业对。目的：通过量化它们的饮食，了解大黑背鸥作为拥有国际重要数量的海鸟繁殖海鸟（英国斯科霍姆）的顶级捕食者的重要性，并确定其在繁殖季节内的变化，以测试配对水平的饮食专业化，并检查饮食差异对繁殖性能的影响。方法：收集并分析了 2017 年 Skokholm 的 26 对繁殖对的反流颗粒，并与繁殖成功有关。结果：对 1035 个颗粒的分析表明，总体而言，大黑背鸥以海鸟（48% – 主要是 Manx Shearwaters Puffinus puffinus）、哺乳动物（38% – 主要是欧洲兔 Oryctolagus cuniculus）、人为废物（7%）和海洋猎物（7%）为食。饮食各不相同，有 18 (73%) 对通才对和 7 (27%) 对专家对（其中 5 对是鸟类专家，2 对是哺乳动物专家）。饮食也随季节变化，但配对水平的饮食多样性在整个繁殖季节是可重复的。饮食多样性与育种成功没有共变。结论：大黑背鸥是斯科霍尔姆的顶级掠食者。成对饮食的变化强调了并非所有个体在捕食方面的贡献都相同。了解这种变异的发生率具有重要的生态意义，特别是在顶级捕食者可能施加强大的自上而下影响的地方。