[eng] Individual specialization refers to individuals using different portions of the total ecological niche of the population. The fact that individuals segregate into different strategies has important implications on ecology, evolution and conservation, which can be even greater than the implications associated to differences among species. Therefore, it is important to know the degree of individual specialization in wild populations and in what dimensions of individuals’ ecological niche it occurs. Similarly, to understand the processes underlying individual specialization, it is key to study intrinsic drivers and environmental conditions that lead individuals to segregate into different portions of the ecological niche. In this thesis, we aimed to delve into the extent of individual specialization in foraging and migration strategies, as well as to provide insights on the intrinsic and extrinsic drivers that shape it. We used two long-lived seabird species, Cory’s (Calonectris borealis) and Scopoli’s (C. diomedea) shearwaters, to study individual specialization in feeding and migratory traits by using stable isotope analyses, global location sensing (GLS) and global positioning system (GPS) loggers. In this thesis, we showed that individuals specialize in several aspects of the ecological niche, namely diet, foraging and wintering grounds, habitat use, daily habits and foraging movements. However, in none of these traits individuals showed high levels of specialization, thus suggesting a stabilizing selection in specialization levels. Regarding intrinsic drivers, our results elucidate that males and females can differ in their degree of individual specialization in diet and foraging movements. These differences were probably driven by a higher use of males on fishery discards. We also showed that the same individual can develop different strategies under different habitats, indicating individual foraging strategies are likely learned with experience when individuals are young and not driven by intrinsic constraints, such as physiological or morphological constraints. Regarding the extrinsic drivers, we provide evidences that resource scarcity is more relevant in driving among-individual variability in foraging movements than resource predictability. Overall, in this thesis we demonstrate that the extent of individual specialization within populations can depend on several intrinsic and extrinsic factors, and that individual specialization is a complex phenomenon that can vary across species and ecological traits. The fact that individuals can develop different strategies independently in different areas indicates a remarkable plasticity that may help them to cope with future natural or anthropogenic changes in the environment
