Low-amplitude vocalisations have only recently begun to receive evolutionary attention, mainly in studies focused on diurnal birds. Owls, as nocturnal and forest-dwelling birds, seem promising for investigating the evolution of these signals, which may play an important role in short-range communication in low-light conditions. Here, we survey the occurrence of low-amplitude vocalisations in owls, combining a synthesis of bibliographic records with original acoustic analyses of audio recordings. We found low-amplitude vocalisations in 63 (25.6%) of all 246 owl species, including 31 species with low-amplitude territorial songs. Low-amplitude songs were typically emitted in response to territorial intrusion, and gradually transitioned into broadcast (high-amplitude) songs. We observed significant acoustic differences between these two song types, which could not be explained by bioacoustic trade-offs alone, suggesting the influence of social selection. We found no evidence that low-amplitude songs are aggressive signals in owls. A recently proposed ‘prospecting signal’ hypothesis, suggesting that low-amplitude songs serve to elicit a response from nearby but unseen rivals, seems compatible with our findings, and deserves further testing. Our study not only advances knowledge about low-amplitude singing in owls, but also proposes owls as a valuable model for further studies of low-amplitude signalling, offering suggestions for future experimental approaches.
Acoustic communication, agonistic behaviour, quiet song, short-range signal, soft song, whisper song