The carotenoid-based colouration that many birds present may honestly reflect the quality of the bearer, as carotenoids have to be obtained through diet. These pigments are important for many physiological functions and individuals could find a trade-off between the allocation of carotenoids for these uses and their accumulation for secondary-sexual and social traits. The Yellow-legged Gull Larus michahellis is an opportunistic species that feeds on a wide spectrum of prey. We studied whether carotenoid-based colouration (bill, red spot, eye-ring) is related to the trophic ecology inferred by the isotopic composition of adults’ feathers at three colonies of this species. The isotopic marker δ13C negatively correlated with carotenoid-based colouration (red spot area). Contrarily to a priori prediction, the marker indicated that higher consumption of landfill scraps and terrestrial prey might lead to a larger red bill spot in adult Yellow-legged Gulls independently of sex. No correlation was detected between isotopic signatures and egg volume in females. the finding that higher consumption of marine prey may imply a smaller red spot supports some results from other avian studies that report that feeding on human-related terrestrial food sources has an apparently positive effect on the adult phenotype. The results may suggest that higher consumption of marine prey may not be as advantageous as supported by previous studies. Nonetheless, the effects of potentially toxic compounds that may come from some feeding sources should be further studied, including any delayed sub-lethal effects on the physiology of adults and, probably, chicks.
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