The usual way of analysing bird-song is by use of a conagraph, which produces a plot of frequency as a function of time, for perods of up to about 2.5 s. This method is fully described by Catchpole (1979), Jellis (1977) and Thorpe (1961). In the present work, a Brüel and Kjaer Narrow Band Spectrum Analyser was used to analyse up to two minutes of bird-song. This instrument produces a fast Fourier analysis of the input in the form of a spectrum of sound pressure level against frequency. Studies of seven Nightjars. Caprimulgus europaeus. show that each individual produces its own characteristic voice spectrum enabling individual birds to be identified with much greater ease than from a sonagram.