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Abstract: The concentrations of Hg were determined in all primary flight feathers from 20 specimens of Buteo buteo and 49 specimens of Accipiter gentilis, which had died in various Wildlife Recovery Centres in Galicia (NW Spain) between 2001 and 2009. The aim of the study was to standardize the feather or feathers that should be analyzed in each species to enable inter-individual comparison of the levels of contamination, taking into consideration any variability in the concentrations of Hg caused by the birds'' molting patterns. For both species the results show a high degree of both inter (e.g. an individual of A. gentilis showed 426ngg−1 of Hg as median and 568ngg−1 as median absolute deviation, MAD) and intra-individual variability (e.g. primary feather number seven – P7 – of all the A. gentilis individuals showed 537ngg−1 of Hg as median and 1129ngg−1 as MAD). Although molting did not have a clear effect on the concentrations of Hg in the flight feathers, it was possible to identify the most representative feather in each individual, taking into account the amount of metal excreted in each feather and the intra-individual variability. For B. buteo use of flight feather P6 is recommended for Hg biomonitoring studies; the amount of Hg excreted to this feather was 509ng as median, which represents the 14% (from 10 to 20%) of the total Hg present in all the primary feathers. For A. gentilis the recommended feather is P7, which has a median of 86ng of Hg (corresponding to 12%, and varying between 5 and 22%). [Copyright &y& Elsevier]