JOURNAL OF AVIAN BIOLOGY www.avianbiology.org Journal of Avian Biology 1 Subject Editor: Yutaka Watanuki Editor-in-Chief: Tomas Alerstam Accepted 11 November 2017 2018: e01463 doi: 10.1111/jav.01463 Niche segregation between similar species will result from an avoidance of competi- tion but also from environmental variability, including nowadays anthropogenic activi- ties. 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, dur- ing 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 dif- fer, and their pellets were comprised mainly by marine fsh species. However, Audouin’s gulls presented higher occurrences of pelagic fsh, while yellow-legged gulls fed more on demersal fsh, insects, and refuse. SIAR mixing models also estimated a higher pro- portion of demersal fsh in the diet of yellow-legged gulls. We also found diferences 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 diferent foraging areas (δ 13 C) and/ or trophic levels (δ 15 N) 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 diferent isotopic niche spaces. Our study suggests that both gull species foraged in association with fsheries 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. Keywords: niche partitioning, isotopic ecology, diet composition Introduction Competition can be one of the major processes structuring avian communities, in par- ticular among sympatric seabird species (Schreiber and Burger 2002, Oro et al. 2009, Moreno et al. 2016). Te degree of segregation among species may change seasonally due Article Seasonal and annual differences in the foraging ecology of two gull species breeding in sympatry and their use of fshery discards J. G. Calado, D. M. Matos, J. A. Ramos, F. Moniz, F. R. Ceia, J. P. Granadeiro and V. H. Paiva J. G. Calado (http://orcid.org/0000-0003-3535-3386) (joana.gomes.calado@gmail.com), D. M. Matos, J. A. Ramos, F. R. Ceia and V. H. Paiva (http:// orcid.org/0000-0001-6368-9579), MARE – Marine and Environmental Sciences Centre, Faculdade de Ciências e Tecnologia, Univ. de Coimbra, Depto de Ciências da Vida, Coimbra, Portugal. – F. Moniz, ICNF – Inst. of Nature Conservation and Forest, RNSCMVRSA, Castro Marim, Portugal. – J. P. Granadeiro, CESAM – Centre for Environmental and Marine Studies, Depto de Biologia Animal, Faculdade de Ciências, Univ. de Lisboa, Lisboa, Portugal. –––––––––––––––––––––––––––––––––––––––– © 2017 Te Authors. Journal of Avian Biology © 2017 Nordic Society Oikos