Research on acoustic communication has focused predominantly on high-amplitude, long-range vocalizations that are easily recognized from a distance by the human ear. However, attention has recently expanded to the more enigmatic low-amplitude (quiet) vocalizations produced during close-proximity interactions associated with a variety of social behaviors such as aggression, courtship, group movements, and predator avoidance. The existence of low-amplitude vocalizations has been identified in a number of individual avian and nonavian species (e.g., crickets, fish, and primates), but how common these signals are within larger taxonomic groups such as orders and classes remains poorly understood. Here, we used the Birds of North America Online archive to perform a survey of the existence and putative functions of low-amplitude vocalizations across 749 species accounts of breeding birds in North America. By searching with keywords such as soft, quiet, low-amplitude, and whispered, we found evidence that 433 species (∼58% of those sampled) produce at least one type of low-amplitude vocalization. Low-amplitude calls were more commonly reported than low-amplitude songs, but both types of vocalizations were thought to be used around twice as often in courtship interactions as in aggressive interactions. Furthermore, 40% of species that sang low-amplitude songs produced at least one song that was thought to be divergent in structure from any high-amplitude songs in the species' repertoire; however, these acoustic differences were predominantly based on each author's auditory perceptions rather than on quantitative data. Collectively, these patterns suggest that low-amplitude vocalizations are common at a broad taxonomic scale, and that low-amplitude songs may be a distinct class of vocal signal. In addition, the observation that low-amplitude vocalizations are produced in a variety of social contexts, including courtship, indicates that these vocalizations should receive greater attention in future studies of communication, sexual selection, and social behavior.
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