Several studies have shown the presence of diverse pollutants in terrestrial ecosystems, but to date there is a dearth of information on their adverse effects on organisms. The eggs of birds have been used to monitor the contamination levels by different xenobiotics, but their usefulness as a tool to investigate their toxicity has been largely neglected. The present chapter highlights the applicability of eggs from a non-conventional model organism for marine–terrestrial ecotoxicology to investigate the toxicity of an emerging environmental pollutant. We investigated the effects of environmental concentrations of perfluorooctane sulfonate (PFOS) on embryos of the yellow-legged gull (Larus michahellis) on diverse phenotypic traits, including morphometric and biochemical endpoints, by an in ovo manipulation approach. None of the endpoints were negatively affected by the injected concentrations of PFOS, independently of the laying order and sex of the embryos, suggesting that current levels of this xenobiotic in yellow-legged gull eggs do not represent a threat for embryo development. Our findings demonstrate that the yellow-legged gull is a reliable model organism for marine–terrestrial ecotoxicology and its eggs are a useful tool to monitor the levels and the toxicity of emerging environmental pollutants.