Evolution of vocal performance and song complexity in island birds

Oceanic islands share distinctive characteristics thought to underlie a set of parallel evolutionary trends across islands and taxonomic groups – including life history traits, morphology and visual signals. To which extent acoustic signals also change in parallel on islands is less clear. Some important processes associated with insularity, such as founder effects and reduced sexual selection, could lead to a decrease in vocal performance and song complexity on islands. In a field-based study, we recorded 11 insular species and their closest mainland relatives. Out of the 11 species pairs, 6 live in the tropics (São Tomé / Mount Cameroon), and 5 in the temperate region (Madeira / southern France). For each species we measured two proxies of vocal performance (song duration and syllable rate) and one proxy of song complexity (syllable diversity). This study did not recover a clear relationship between the island environment and song traits. If as expected, syllable rate was lower in island species than in their mainland counterparts, the two other proxies showed no clear island-mainland pattern of divergence. Several factors may explain the absence of reduction for song duration and syllable diversity. Among those, relaxation of interspecific competition on islands may have led to an increase in syllable diversity, or correlations between song variables may have constrained song evolution. More studies on island species are needed to draw a better picture of divergence patterns and go beyond the confounding ecological factors that could explain peculiar song characteristics in islands.

Methods
Field based study. Song recordings.

大洋岛屿具有独特的特征，这些特征被认为是跨岛屿及不同分类类群（taxonomic groups）的一系列平行演化趋势的基础——涵盖生活史性状（life history traits）、形态学与视觉信号。然而，声学信号在岛屿环境中是否也会呈现平行演化，目前仍不甚明确。与岛屿栖息相关的若干关键过程，例如奠基者效应（founder effect）与性选择（sexual selection）压力减弱，可能会导致岛屿物种的鸣唱表现（vocal performance）下降、鸣唱复杂度（song complexity）降低。
本研究通过野外实地考察，录制了11个岛屿物种及其近缘的大陆物种类群的鸣唱。在这11个物种类群对中，6个类群分布于热带区域（圣多美岛/喀麦隆山），剩余5个类群分布于温带区域（马德拉岛/法国南部）。针对每个物种，我们测定了两项鸣唱表现（vocal performance）的替代指标：鸣唱时长（song duration）与音节速率（syllable rate），以及一项鸣唱复杂度（song complexity）的替代指标：音节多样性（syllable diversity）。
本研究未发现岛屿环境与鸣唱性状之间存在明确的关联。尽管按照预期，岛屿物种的音节速率应低于其大陆近缘类群，但其余两项替代指标并未呈现出清晰的岛屿-大陆分化模式。有若干因素可解释鸣唱时长与音节多样性未出现下降的现象。其中，岛屿上种间竞争压力的松弛可能提升了音节多样性，或是鸣唱变量间的相关性限制了鸣唱演化。未来需开展更多针对岛屿物种的研究，以更清晰地刻画其鸣唱性状的分化模式，并厘清可能解释岛屿物种独特鸣唱特征的混杂生态因素。
研究方法：野外实地考察与鸣唱录制。