Mercury concentration levels were determined in the kidneys of 131 specimens of 42 species of birds with different food preferences. The highest concentrations were found in piscivorous species such as the goosander, great white egret, white-tailed eagle and black stork. The following sequence was identified for concentration levels in kidneys of birds in different trophic guilds: piscivorous > small mammal eating > bird eating > aquatic invertebrate eating > terrestrial invertebrate eating > herbivorous > omnivorous. The level of renal mercury concentration was found to be significantly lower in young birds than in adults. Increased mercury concentration was also registered in the kidneys of birds from Warsaw, the capital of Poland.