Repeatability in isotopic signatures is linked to consistent individual migration strategies and individual specialization in a long-distance migratory seabird
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- Institutt for biologi 
Animal populations can consist of specialized individuals with different foraging strategies. There could be several advantages for specialized individuals, such as higher foraging efficiency, breeding success and survival. Individuals may follow the same foraging strategies over several years, showing a high foraging consistency. In this seven-year study, we investigated the variation of foraging ecology in individuals of the Arctic skua (Stercorarius parasiticus) wintering in six different areas. We analyzed stable isotopes of nitrogen and carbon (δ15N and δ13C) in feathers to define their isotopic niche during winter which indicates their ecological niche. Combining stable isotope analyses with migration tracks from light-level loggers (Global Location Sensing loggers) of 58 individuals in three breeding colonies in Svalbard and Northern Norway from 2010 to 2016, we describe variation in foraging ecology between wintering areas, among individuals and within individuals among years. The core of the study design is therefore repeated sampling of feathers and tracking of the same individuals across several years. Individuals wintered in one of six areas; Benguela current, Caribbean, Canary current, Falkland current, Gulf of Guinea and Mediterranean, and displayed consistent migration strategies across years. Isotopic niche differed among the six wintering areas and differed also among individuals within wintering areas. The high individual repeatability for both δ15N and δ13C values, after controlling for wintering area, indicates consistent individual foraging strategies and a high level of specialization. This study is one of few that shows consistency in the winter foraging ecology in seabirds. The individual consistency in foraging ecology of Arctic skuas raises the question if birds are able to adapt their feeding behavior to changes in forage distribution due to climate change.