Songbirds use acoustic signals (songs) to defend territories and attract mates. Proximate and ultimate causes of variations in song structure have been extensively studied, but vocal amplitude is often overlooked and rarely measured. This lack of data is unfortunate because signal amplitude is crucial in communication. In bird song, it plays an important role not only for signal transmission, but also in territorial behaviours and mate choice. However, since a long time it has been assumed that differences in song amplitude between species are mainly due to variation in body size, with bigger species producing louder songs. We present comparative data from twelve common European songbird species that challenge this view. We found marked variation in song amplitude but this variation was not related to body size. Instead, song amplitude varied with small-scale social and environmental conditions (such as time of day or the presence of competitors). If body size affects amplitude at all, then the effect is so small that it is buried by variation due to these or other factors. Generally, our data highlight the importance of individual plasticity in song performance, and hint at a complex interplay between the constraints and functions of bird song.