A survey of the occurrence and possible functions of low-amplitude songs in owls

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.

低振幅发声（low-amplitude vocalisations）直到近年才开始受到演化生物学领域的关注，相关研究多聚焦于昼行性鸟类。鸮类（owls）作为夜行性森林栖息鸟类，是研究此类信号演化的极具潜力的类群——这类信号可能在低光照环境下的近距离通讯中发挥关键作用。本研究综合梳理文献记录，并结合对录音样本的原创声学分析，系统调查了鸮类中低振幅发声的存在情况。我们在全部246种鸮类中，发现63种（占比25.6%）存在低振幅发声，其中包含31种拥有低振幅领域鸣唱的物种。低振幅鸣唱通常是针对领域入侵行为发出的，并会逐渐过渡为宣告式（high-amplitude）鸣唱。我们观测到这两类鸣唱之间存在显著声学差异，且该差异无法仅用生物声学权衡机制解释，暗示了社会选择（social selection）的影响。我们未发现证据表明鸮类的低振幅鸣唱属于攻击信号。近期提出的‘探查信号（prospecting signal）’假说认为，低振幅鸣唱的作用是唤起附近未现身竞争者的回应，该假说与我们的研究结果相符，值得开展进一步验证。本研究不仅拓展了我们对鸮类低振幅鸣唱的认知，还提出将鸮类作为研究低振幅信号通讯的优质模型类群，并为未来的实验研究方向提供了参考建议。