Over recent decades, deer populations have reached densities exceeding their natural range of variability in several regions worldwide. Abundant deer populations can change the composition and structure of understory plant communities in forests and trigger trophic cascades affecting animal communities. To investigate such indirect effects on boreal forest songbirds we compared the understory vegetation and songbird communities of balsam fir stands on Anticosti Island (Quebec, Canada), where white-tailed deer (Odocoileus virginianus) were introduced, to those of the neighbouring Mingan Archipelago, where deer are absent. Mean cover of trees at 0–2 m height was on average sevenfold higher on Mingan than on Anticosti, and songbirds dependent on the understory were more common on Mingan than on Anticosti. Songbird community composition was more variable on Mingan, suggesting that the opening of the forest understory on Anticosti through intensive browsing homogenized the songbird community by reducing the occurrence of understory-dependent species. These indirect effects on songbirds were similar, although less dramatic, than those described in temperate forests. We interpret this as a consequence of lower productivity and complexity in understory vegetation of boreal forests. Our results emphasize the negative effects of abundant deer populations on animal diversity and underline the role played in the dynamics of communities by top down factors that regulate herbivore populations.