Over the twentieth century, bird vocalizations have become a key site to study biological mechanisms of evolution, behavior and perception. For the study and collecting of these vocalizations, in the field-site as in the laboratory, biologists often relied to a large extent on their listening skills. This paper traces the changing organization and status of (mediated) listening practices as a way of gaining and producing new knowledge, as well as the recognition of listening as a marker of a specific professional expertise. Drawing on analysis of ornithological journals, archived correspondence and field notes, this presentation follows the introduction of several new technologies and instruments for the recording and analysis of acoustic phenomena in the interwar period, in both Britain and the United States. By the late nineteenth century, the practice of aural field observation emerged largely outside academic ornithology, which was focused primarily on museum taxonomy. In an attempt to legitimize the sonic as an entrance to gain new knowledge, amateur naturalists and some professional ornithologists presented their expert hearing as informed by systematic attention and musical experience. Ornithologists fashioned themselves as ‘scientist-musicians’ and prided themselves on the musical ear with which they recorded and analyzed natural sounds. But with increased interest in aural observation, naturalists became critically divided over the presumed adequacy of musical knowledge. The introduction of the electrical microphone, phonograph and sound-camera in field-recording in the 1930s by institutes such as Cornell University and the British BBC, however, largely side-tracked the issue of notation. At the same time, the ‘mechanical ear’ changed the epistemic status and organization of listening in scientific work. Not only did it expose unmediated human listening as ‘selective’ and ‘subjective’; the expensive and technically complex equipment also re-located listening expertise with just a small group of professional recordists and ornithologists, who learned to judge and discuss their subject now in acoustic rather than musical terms. Acquired knowledge of sound engineering in turn influenced the way the field was being experienced, as the ‘unselective’ mechanical ear picked up so much interference and noise that scientists had to learn and develop new techniques and instruments for selective and focused listening.