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Este artículo contiene 12 páginas, 5 figuras, 1 tabla. ; Many generalist species are composed of individuals varying in the size of their realized niches within a population. To understand the underlying causes and implications of this phenomenon, repeated samplings on the same individuals subjected to different environmental conditions are needed. Here, we studied individual specialization of feeding strategies in breeding and non-breeding grounds of Cory's shearwaters (Calonectris borealis) for 2–8 years, and its relationship with fitness. Individuals were relatively flexible in non-breeding destinations, but specialized in diet, habitat use and daily activity across years. Daily activity was also consistent throughout the year for the same individual, suggesting that it is driven by individual constraints, whereas individual diet and habitat use changed between breeding and non-breeding grounds, indicating that these specializations may be learned at each area. Moreover, individuals that were intermediate specialized in their diet tended to show higher breeding success than those with weakly and highly specialized diets, suggesting stabilizing selection. Overall, this study suggests that the development of individual specialization is more flexible than previously thought, i.e. it emerges under specific environmental conditions and can develop differently when environmental conditions vary. However, once established, individual specialization may compromise the ability of individuals to cope with environmental stochasticity. ; L. Zango was supported by a PhD grant from the Spanish government (Formación de Profesorado Universitario; FPU15/01927). J.M. Reyes-González was supported by a PhD grant from the Spanish government (Formación de Profesorado Universitario; AP2009–2163). T. Militão was supported by a Ph.D. Grant (SFRH/BD/47467/2008) from Fundação para a Ciência e Tecnologia (Portugal). R. Ramos was supported by postdoctoral contracts of the PLEAMAR (2017/2349) and Ramón y Cajal (RYC-2017–22055) programs from the European Maritime and Fisheries Fund (EMFF) and the Spanish Ministerio de Educación y Ciencia, Ministerio de Ciencia e Innovación (MINECO), respectively. Z. Zajková was supported by a PhD Grant from University of Barcelona (APIF/2012). We would like to thank everyone that participated in fieldwork in Veneguera and Montaña Clara from 2007 to 2015: R. Arizmendi, N. Pérez, A. Vulcano, M. Codina, J. Moreno, L. Stefan, E. Gómez-Díaz, J. Navarro, M. López, E. Rodríguez, I. De Diego, A. Soriano, M. Heus, M. Pastor, R. Manzano, H. Dinis, C. Astals, M. Smyrli, P. Serrano, J. Ferrer-Obiol, N. Gomes and I. García, I. Díaz, M. Cruz and C. Soria. Pascual Calabuig and Loly Estévez provided invaluable logistic support every year in Gran Canaria. We are especially grateful with V. Morera-Pujol, Daniel Sol and Pascual López-López for their support and contribution at different stages of this work and Fernanda De Felipe for the molecular sexing analyses. This study was funded by MINECO, Ministerio de Economía y Competitividad (CGL2006-01315/BOS, CGL2009-11278/BOS, CGL2013-42585-P and CGL2016-78530-R) from the Spanish government and Fondos FEDER, and Research Executive Agency (European Commission, FP7- PEOPLE-2013-CIG, 618841). ; Peer reviewed