Blood and feathers are the most targeted tissues for isotopic investigations in avian ecology, primarily because they can be easily and non-destructively sampled on live individuals. Comparing blood and feather isotopic ratios can provide valuable information on dietary shifts, trophic specialization and migration patterns, but it requires a good knowledge of the isotopic differences between the two tissues. Here, δsup.13C and δsup.15N values of whole blood (in blood cells of a few species) and simultaneously grown body feathers were measured in seabird chicks to quantify the tissue-related isotopic differences. Seabirds include 27 populations of 22 wild species that were sampled in 2000-2008, and a review of the literature added 8 groups (including adult birds) to the analysis. The use of a large data set that overall encompasses wide δsup.13C and δsup.15N ranges allowed us to depict for the first time accurate relationships between blood and feather isotopic ratios across avian taxa. Blood was impoverished in sup.13C and generally in sup.15N compared with feathers. Both mean δsup.13C and δsup.15N values of feathers and blood were highly positively and linearly related feather δsup.13C = 0.972 (±0.020) blood δsup.13C + 0.962 (+ or -0.414), and feather δsup.15N = 1.014 (±0.056) blood + 0.447 (±0.665), respectively; both P < 0.0001. The regressions should be applied to mathematically correct feather or whole blood δsup.13C and δsup.15N values when comparing isotopic ratios within and between ecological studies on birds.