The study of the development of song in birds has provided a wealth of information regarding basic principles of neural, endocrinological, and behavioral development. Much of the success that has been enjoyed in this field is due to the solid base of observations collected for hundreds of years. People have long taken pleasure in ob serving bird behavior and in listening to the songs and calls of birds of various species. This early wealth of observational data was supplemented rapidly with the development of studio and por table sound-recording equipment that permitted the recording of songs for future enjoyment and study. With the development of the first spectral analysis devices, it became possible to produce a visual display of sounds, which could be sub jected to precise measurements and statistical analyses. The observations and recordings were especially valuable because many of them were made in the field while the birds were engaged in their normal behaviors, and they provided information regarding the context of the ongo ing behaviors while the vocalizations occurred. These technological advances made it possible to understand the functional significance of vocal izations in the presence of alien and conspecific intruders, neighbors, mates, and young.