Mercury is a toxic heavy metal that can cause obvious physiological and reproductive problems in animals. Very little is known, however, about its subtle behavioral effects. We examined whether birds that inhabited mercury-contaminated sites exhibited differences in singing behavior compared with birds at uncontaminated reference sites nearby. We recorded the songs of 3 oscines, the Carolina Wren (Thryothorus ludovicianus), House Wren (Troglodytes aedon), and Song Sparrow (Melospiza melodia), and 1 suboscine, the Eastern Phoebe (Sayornis phoebe). Spectrographic analysis revealed that songs of oscines living on contaminated sites contained a lower diversity of note types and were sung at lower tonal frequencies than songs of birds on reference sites. Additionally, both species of wren tended to sing shorter songs. By contrast, the songs of Eastern Phoebes did not differ between contaminated and reference sites, suggesting that mercury may affect singing behavior only in species that learn their songs. Such alterations in song could have important implications for the fitness of songbirds in polluted areas. Our results highlight the importance of considering behaviors in evaluations of contaminant effects.