Competition-driven feeding niche separation is assumed to be an important driver of the morphological divergence of co-occurring animal species. However, despite a strong theoretical background, empirical studies showing a direct link between competition, diet divergence and specific morphological adaptations are still scarce. Here we studied the early steps of competition-driven eco-morphological divergence in two closely related passerines: the common nightingale ( Luscinia megarhynchos ) and the thrush nightingale ( Luscinia luscinia ). Our aim was to test whether previously-observed divergence in bill morphology and habitat in sympatric populations of both species is associated with dietary niche divergence. We collected and analysed data on (1) diet, using both DNA metabarcoding and visual identification of prey items, (2) habitat use, and (3) bill morphology in sympatric populations of both nightingale species. We tested whether the species differ in diet composition and whether there are any associations among diet, bill morphology and habitat use. We found that the two nightingale species have partitioned their feeding niches, and showed that differences in diet may be partially associated with the divergence in bill length in sympatric populations. We also observed an association between bill length and habitat use, suggesting that competition-driven habitat segregation could be linked with dietary and bill size divergence. Our results suggest that interspecific competition is an important driver of species’ eco-morphological divergence after their secondary contact, and provide insight into the early steps of such divergence in two closely related passerine species. Such divergence may facilitate species coexistence and strengthen reproductive isolation between species, and thus help to complete the speciation process.

中文翻译：

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追踪两个姐妹雀类物种竞争驱动的生态形态差异的早期步骤

竞争驱动的饲养生态位分离被认为是共生动物物种形态差异的重要驱动因素。然而，尽管有强大的理论背景，但表明竞争、饮食差异和特定形态适应之间存在直接联系的实证研究仍然很少。在这里，我们研究了两种密切相关的雀科动物中竞争驱动的生态形态差异的早期步骤：普通夜莺（ Luscinia megarhynchos ）和画眉夜莺（ Luscinia luscinia ）。我们的目的是测试先前观察到的两种物种同域种群中喙形态和栖息地的差异是否与饮食生态位差异有关。我们收集并分析了 (1) 饮食方面的数据，使用 DNA 元条形码和猎物的视觉识别，(2) 栖息地使用，(3) 两种夜莺同域种群的喙形态。我们测试了该物种的饮食成分是否不同，以及饮食、喙形态和栖息地使用之间是否存在任何关联。我们发现这两种夜莺物种已经划分了它们的觅食生态位，并表明饮食的差异可能部分与同域种群中喙长度的差异有关。我们还观察到喙长度和栖息地使用之间的关联，这表明竞争驱动的栖息地隔离可能与饮食和喙大小的差异有关。我们的研究结果表明，种间竞争是物种二次接触后生态形态分化的重要驱动因素，并为深入了解两种密切相关的雀形目物种的这种分化的早期步骤提供了见解。