Blood and feathers are the two most targeted avian tissues for environmental biomonitoring studies, with mercury (Hg) concentration in blood and body feathers reflecting short and long-term Hg exposure, respectively. In this work, we investigated how Hg isotopic composition (e.g. δ202Hg and Δ199Hg) of blood and feathers from either seabird chicks (skuas, n=40) or adults (penguins, n=62) can accurately provide information on exposure to Hg in marine ecosystems. Our results indicate a strong correlation between blood and feather Hg isotopic values for skua chicks, with similar δ202Hg and Δ199Hg values in the two tissues (mean difference: -0.01±0.25 ‰ and -0.05±0.12 ‰, respectively). Since blood and body feathers of chicks integrate the same temporal window of Hg exposure, this suggests that δ202Hg and Δ199Hg values can be directly compared without any correction factors within and between avian groups. Conversely, penguin adults show higher δ202Hg and Δ199Hg values in feathers than in blood (mean differences: 0.28±0.19 ‰ and 0.25±0.13 ‰), most likely due to tissue-specific Hg temporal integration. Since feathers integrate long-term (i.e. the inter-moult period) Hg accumulation, whereas blood exhibits short-term (i.e. seasonal) Hg exposure in adult birds, the two tissues provide complementary information of trophic ecology at different time scales.