Data from: Are vocal characteristics related to leadership patterns in mixed-species bird flocks?

What structures the organization of mixed-species bird flocks, so that some ‘nuclear’ species lead the flocks, and others follow? Previous research has shown that species actively listen to each other, and that leaders are gregarious; such gregarious species tend to make contact calls and hence may be vocally conspicuous. Here we investigated whether vocal characteristics are associated with leadership, using a global dataset of mixed-species flock studies and recordings from sound archives. We first asked whether leaders are different from following or occasional species in flocks in the proportion of the recordings that contain calls (n=58 flock studies, 145 species), and especially alarm calls (n=111 species). We found that leaders tended to have a higher proportion of their vocalizations that were calls than occasional species, and both leaders and following species had a significantly higher proportion of their calls rated as alarms compared to occasional species. Next, we investigated the acoustic characteristics of flock participants’ calls, hypothesizing that leaders would make more calls, and have less silence on the recordings. We also hypothesized that leaders’ calls would be simple acoustically, as contact calls tend to be, and thus similar to each other, as well as being detectable, in being low frequency and high bandwidth. The analysis (n=45 species, 169 recordings) found that only one of these predictions was supported: leading species were less often silent than following or occasional species. Unexpectedly, leaders’ calls were less similar to each other than occasional species. The greater amount of information available and the greater variety of that information support the hypothesis that leadership in flocks is related to vocal communication. We highlight the use of sound archives to ask questions about behavioral and community ecology, while acknowledging some limitations of such studies.

是什么塑造了混合物种种鸟群（mixed-species bird flocks）的组织模式，使得部分‘核心物种（nuclear species）’引领鸟群，其余物种则紧随其后？既往研究表明，不同鸟类物种会主动聆听同类及他种个体的鸣声，且引领者多为群居性物种；这类群居性物种通常会发出联络鸣唱（contact calls），因此在鸣声上往往更为显著。本研究借助全球范围内的混合物种种鸟群研究数据集与声音档案库（sound archives）中的鸣声录音，探究鸣声特征是否与物种的引领地位相关。我们首先提出首个问题：在鸟群中，引领物种、跟随物种与偶见物种的鸣声录音中，包含鸣唱的比例是否存在差异（共纳入58项鸟群研究、145个物种），尤其是警戒鸣唱（alarm calls）的相关比例（共纳入111个物种）？研究结果显示，相较于偶见物种，引领物种的鸣唱占其所有鸣声的比例更高；且与偶见物种相比，引领物种与跟随物种的鸣唱中被归类为警戒鸣唱的比例均显著更高。随后，我们针对鸟群参与者的鸣唱声学特征展开研究，提出两项假设：其一，引领物种会发出更多鸣唱，且录音中的静默时长更短；其二，引领物种的鸣唱在声学上更为简单（正如联络鸣唱的典型特征），因此彼此间的鸣唱更为相似，同时因其频率较低、带宽（bandwidth）较宽而更易被检测到。本次分析共纳入45个物种、169条录音，结果显示仅一项假设得到验证：引领物种出现静默的频率低于跟随物种与偶见物种。出乎预料的是，引领物种的鸣唱彼此间的相似度反而低于偶见物种。可获取的鸣声信息量更大、信息种类更多的结果，支持了‘鸟群中的引领地位与鸣声通讯相关’这一假说。本研究强调可借助声音档案库开展行为生态学与群落生态学（behavioral and community ecology）相关问题的探究，同时也指出了这类研究存在的部分局限。