Data from: Interspecific competition promotes habitat and morphological divergence in a secondary contact zone between two hybridizing songbirds

Interspecific competition is assumed to play an important role in the ecological differentiation of species and speciation. However, empirical evidence for competition’s role in speciation remains surprisingly scarce. Here we studied the role of interspecific competition in the ecological differentiation and speciation of two closely related songbird species, the Common Nightingale (Luscinia megarhynchos) and the Thrush Nightingale (L. luscinia). Both species are insectivorous and ecologically very similar. They hybridize in a secondary contact zone, which is a mosaic of sites where both species co-occur (syntopy) and sites where only one species is present (allotopy). We analyzed fine-scale habitat data for both species in both syntopic and allotopic sites and looked for associations between habitat use and bill morphology, which have been previously shown to be more divergent in sympatry than in allopatry. We found that the two nightingale species differ in habitat use in allotopic sites, where L. megarhynchos occurred in drier habitats and at slightly higher elevations, but not in syntopic sites. Birds from allotopic sites also showed higher interspecific divergence in relative bill size compared to birds from syntopic sites. Finally, we found an association between bill morphology and elevation. Our results are consistent with the view that interspecific competition in nightingales has resulted in partial habitat segregation in sympatry, and that the habitat-specific food supply has in turn very likely led to bill size divergence. Such ecological divergence may enhance prezygotic as well as extrinsic postzygotic isolation and thus accelerate the completion of the speciation process.

种间竞争被认为在物种的生态分化与物种形成过程中发挥关键作用。然而，有关竞争在物种形成中所起作用的实证证据却出奇地匮乏。本研究针对两种亲缘关系密切的鸣禽——普通夜莺（Luscinia megarhynchos）与鸲夜莺（L. luscinia），探究了种间竞争在其生态分化与物种形成中的作用。二者均为食虫性鸟类，生态特征高度相似，且在次生接触带发生杂交；该接触带呈现镶嵌状分布格局：既有两物种共同出现的同域共存位点（syntopy），也有仅单一物种存在的异域位点（allotopy）。我们对同域共存位点与异域位点中两种夜莺的生境数据展开精细尺度分析，并探究了生境利用与喙部形态之间的关联——此前研究已表明，喙部形态的种间分化在同域种群中较异域种群更为显著。研究结果显示，两种夜莺在异域位点的生境利用存在显著差异：普通夜莺偏好更为干燥的生境，且分布于稍高海拔区域；但在同域共存位点中，二者的生境利用并无明显分化。相较于同域共存位点的种群，异域位点的种群在相对喙部尺寸上的种间分化程度同样更高。此外，我们还发现喙部形态与海拔高度存在显著关联。本研究结果支持如下假说：夜莺的种间竞争已促使同域种群产生部分生境分化，而生境特异性的食物资源反过来又极有可能推动了喙部尺寸的分化。此类生态分化可增强合子前隔离与外在合子后隔离，进而加速物种形成过程的完成。