Although scientific work is widely considered a predominantly visual endeavor, sound has been not only an object of scientific investigation but also an epistemic tool. My project investigates this double role of listening by examining how scientists have sought to render the sounds of the natural world into a legitimate source of scientific knowledge. Starting in the late nineteenth century, ornithologists and field biologists attempted to scientifically study birds’ acoustic behavior both in the field and in the laboratory, serving purposes of field recognition as well as the study of taxonomic difference, learning processes, or population dynamics. In doing so, they drew on a range of media for transcribing and recording sounds, from musical notations and gramophones to spectrograms. How did sound recording become a scientific technique? How did ornithologists employ their ears in making sense of what they studied, and how did such practices of mediated and unmediated listening generate new acoustical and behavioral knowledge? Finally, how did these kinds of listening come to be legitimized as authoritative scientific practices? The project traces a history of scientific listening between 1880 and 1980, across locales that include dispersed field sites and institutions such as the Cornell Laboratory of Ornithology, the British Broadcasting Corporation, and the Cambridge University department of zoology. It shows how scientific records and bio-acoustical knowledge ultimately came into existence through ornithologists’ multifarious collaborations with amateur birdwatchers, hobbyist sound hunters, recording engineers, public broadcasters, and musicians.