Data from: Deciphering information encoded in birdsong: male songbirds with fertile mates respond most strongly to complex, low-amplitude songs used in courtship

Research on the function of acoustic signals has focused on high-amplitude, long-range song (LRS) and largely ignored low-amplitude songs produced by many species during close-proximity, conspecific interactions. Low-amplitude songs can be structurally identical to LRS (soft LRS) or they can be widely divergent, sharing few spectral and temporal attributes with LRS (short-range song (SRS)). SRS is often more complex than LRS and is frequently sung by males during courtship. To assess function, we performed two playback experiments on males of a socially monogamous songbird. We compared responses of males whose mates were fertile or non-fertile to differences in song structure (SRS v. LRS and soft LRS), amplitude (SRS and soft LRS v. LRS), and tempo (slow v. fast SRS). Males responded more strongly to SRS than to LRS or soft LRS, indicating that song structure had a greater effect on response than song amplitude. SRS tempo did not detectably affect male response. Importantly, males responded more strongly to SRS when their mates were fertile, presumably because hearing SRS can indicate that a male's mate is being courted by an intruding male and a strong response can deter extra-pair competitors. We conclude that low-amplitude songs can function in both inter- and intra-sexual communication and should receive greater attention in future studies of mate choice and male-male competition.学界关于声学信号功能的研究长期聚焦于高振幅长距离鸣唱（long-range song, LRS），却在很大程度上忽略了诸多物种在近距离同种个体互动中发出的低振幅鸣唱。此类低振幅鸣唱在结构上可与长距离鸣唱（LRS）完全一致（此类鸣唱被称为软长距离鸣唱（soft LRS）），也可与LRS差异显著，与LRS几乎无共同的频谱与时域特征（此类鸣唱被称为短距离鸣唱（short-range song, SRS））。短距离鸣唱（SRS）通常比长距离鸣唱更为复杂，且常由雄性在求偶过程中发出。为探明此类鸣唱的功能，我们针对一种社会性单配制鸣禽的雄性个体开展了两项回放实验。我们分别比较了配偶处于可繁殖期与非可繁殖期的雄性个体，对不同鸣唱结构（短距离鸣唱SRS、长距离鸣唱LRS与软长距离鸣唱soft LRS）、鸣唱振幅（低振幅的SRS与soft LRS相较于高振幅的LRS）以及鸣唱节奏（慢速SRS与快速SRS）的反应差异。实验结果显示，雄性个体对短距离鸣唱的反应强度显著高于长距离鸣唱与软长距离鸣唱，表明鸣唱结构对反应的影响程度大于鸣唱振幅。短距离鸣唱的节奏则未对雄性的反应产生可检测到的影响。尤为关键的是，当雄性个体的配偶处于可繁殖期时，其对短距离鸣唱的反应更为强烈——这或许是因为感知到短距离鸣唱，意味着雄性的配偶正受到外来雄性的求偶挑逗，而强烈的反应能够威慑额外配对竞争者。综上，我们认为低振幅鸣唱可同时参与性间与性内通讯，且在未来关于配偶选择以及雄性间竞争的研究中应受到更多关注。