Complexity, Predictability and Time Homogeneity of Syntax in the Songs of Cassin’s Vireo (Vireo cassinii)

Many species of animals deliver vocalizations in sequences presumed to be governed by internal rules, though the nature and complexity of these syntactical rules have been investigated in relatively few species. Here I present an investigation into the song syntax of fourteen male Cassin’s Vireos (Vireo cassinii), a species whose song sequences are highly temporally structured. I compare their song sequences to three candidate models of varying levels of complexity–zero-order, first-order and second-order Markov models–and employ novel methods to interpolate between these three models. A variety of analyses, including sequence simulations, Fisher’s exact tests, and model likelihood analyses, showed that the songs of this species are too complex to be described by a zero-order or first-order Markov model. The model that best fit the data was intermediate in complexity between a first- and second-order model, though I also present evidence that some transition probabilities are conditioned on up to three preceding phrases. In addition, sequences were shown to be predictable with more than 54% accuracy overall, and predictability was positively correlated with the rate of song delivery. An assessment of the time homogeneity of syntax showed that transition probabilities between phrase types are largely stable over time, but that there was some evidence for modest changes in syntax within and between breeding seasons, a finding that I interpret to represent changes in breeding stage and social context rather than irreversible, secular shifts in syntax over time. These findings constitute a valuable addition to our understanding of bird song syntax in free-living birds, and will contribute to future attempts to understand the evolutionary importance of bird song syntax in avian communication.诸多动物类群均会以序列形式发出鸣唱信号，这类序列被认为受内在规则支配；不过迄今为止，针对这类句法规则的本质与复杂度的研究仅覆盖了相对较少的物种。本研究针对14只雄性卡森莺（Vireo cassinii）的鸣唱句法展开分析，该物种的鸣唱序列具有高度的时间结构化特征。研究将其鸣唱序列与三种复杂度各异的候选模型——零阶、一阶与二阶马尔可夫（Markov）模型——进行比对，并采用创新方法在这三类模型间实现插值推演。本次研究采用序列模拟、费希尔精确检验（Fisher’s exact test）以及模型似然分析等多种分析手段，结果显示该物种的鸣唱复杂度远超零阶或一阶马尔可夫模型的拟合能力。拟合效果最优的模型复杂度介于一阶与二阶马尔可夫模型之间，但本研究同时发现，部分转换概率的条件依赖于最多前序三个鸣唱乐句。此外，整体而言该物种的鸣唱序列预测准确率超过54%，且预测准确性与鸣唱发出速率呈正相关关系。针对句法时间同质性的评估结果显示，不同鸣唱乐句类型间的转换概率在时间维度上整体保持稳定，但有证据表明在繁殖季内及繁殖季之间，句法存在小幅变化；本研究认为这一现象反映的是繁殖阶段与社会环境的改变，而非句法随时间发生的不可逆长期演变。本研究结果为我们理解野生鸟类鸣唱句法提供了宝贵的新认知，也将为后续探究鸟类鸣唱句法在鸟类通讯中的进化意义提供助力。