Data from: The relative response of songbirds to shifts in song amplitude and song minimum frequency

Anthropogenic noise presents a problem for acoustic communication in animal taxa around the world. Many animals respond by modifying their acoustic signals, sometimes along multiple axes, such as song structure, redundancy, or amplitude. To date, no study has assayed the relative response of animals to multiple axes of signal variation, such as song structure and song amplitude, associated with anthropogenic noise levels. To investigate the impact of multiple potential adaptations to anthropogenic noise on targeted receivers, we manipulated song amplitude and song minimum frequency of white-crowned sparrow (Zonotrichia leucophrys) songs. We used a 2 × 2 factorial design of playback experiments to measure male territorial responses to songs that were relatively quiet or loud in relation to typical Z. leucophrys songs and with lower or higher minimum frequencies within the range of natural Z. leucophrys songs. Males responded more strongly to louder songs than to quieter songs and more strongly to lower than to higher minimum frequency songs, with the strongest responses to louder songs with relatively lower minimum frequencies. These results indicate that whether or not increasing amplitude or increasing minimum frequency is more effective at increasing signal transmission distance in anthropogenic noise, increases in signal amplitude increase signal salience in male–male interactions. Thus in the context of territoriality and sexual selection, an increase in song amplitude can compensate for losses in signal salience due to higher minimum frequency. An increase in only song minimum frequency in the context of low frequency anthropogenic noise could be maladaptive.

人为噪音（anthropogenic noise）对全球各类动物类群的声学通讯造成不利影响。许多动物会通过调整自身声学信号作出响应，有时会在多个维度上进行调整，例如鸣声结构、信号冗余度或振幅。截至目前，尚无研究针对与人为噪音水平相关的多维度信号变异（如鸣声结构与鸣声振幅），开展动物相对响应的检测。为探究多种针对人为噪音的潜在适应性对目标接收者的影响，我们对白冠带鹀（Zonotrichia leucophrys）鸣声的振幅与最低频率进行了操控。我们采用2×2析因设计（2 × 2 factorial design）的回放实验（playback experiments），测定雄性个体对两类鸣声的领地响应（territorial responses）：这些鸣声相较于白冠带鹀的典型鸣声，分别为相对轻柔与相对响亮组，且最低频率处于该物种自然鸣声范围内的较低或较高区间。实验结果显示，雄性个体对响亮鸣声的响应强度高于轻柔鸣声，对最低频率较低的鸣声响应强度高于较高者，其中对最低频率相对较低的响亮鸣声响应最强。上述结果表明，无论在人为噪音环境中，提升振幅或是提升最低频率何者更能有效增加信号传播距离，信号振幅的提升均能增强雄性间互动（male–male interactions）中的信号显著性。因此在领地性与性选择（sexual selection）的背景下，鸣声振幅的提升可弥补因最低频率升高带来的信号显著性损失。在低频人为噪音环境下仅提升鸣声最低频率的策略，可能是适应不良的（maladaptive）。