1. The vocal performance of different species of songbird was compared by measuring the maximum total sound power produced during normal song. This varied from 10 mW/Kg body weight in the linnet Acanthus cannabina and the whitethroat Sylvia communis to 870 mW/Kg in the song-thrush Turdus philomelos.
2. In comparison, the performance of the chicken Gallus domesticus during crowing was approximately 60 mW/Kg.
3. There was some evidence that performance was related to size in the songbirds as a group, the smaller bird being less effective than the larger.
4. Differences in performance are discussed in relation to the presence or absence of intrinsic muscles in the syrinx and to possible effects of scale on the efficiency of the fundamental sound-producing process.