Migratory divides, hybrid zones between populations that use different seasonal migration routes, are hypothesised to contribute to speciation. Specifically, relative to parental species, hybrids at divides are predicted to exhibit (1) intermediate migratory behaviour and (2) reduced fitness as a result. We provide the first direct test of the second prediction here with one of the largest existing avian tracking datasets, leveraging a divide between Swainson's thrushes where the first prediction is supported. Using detection rates as a proxy for survival, our results supported the migratory divide hypothesis with lower survival rates for hybrids than parental forms. This finding was juvenile‐specific (vs. adults), suggesting selection against hybrids is stronger earlier in life. Reduced hybrid survival was not explained by selection against intermediate phenotypes or negative interactions among phenotypes. Additional work connecting specific features of migration is needed, but these patterns provide strong support for migration as an ecological driver of speciation.

中文翻译：

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鸣禽之间迁徙鸿沟导致杂交存活率降低

迁徙鸿沟，即使用不同季节性迁徙路线的种群之间的混合区，被认为有助于物种形成。具体来说，相对于亲本物种，分叉处的杂种预计会表现出（1）中间的迁徙行为和（2）因此适应度降低。我们利用现有最大的鸟类跟踪数据集之一，利用支持第一个预测的斯温森画眉之间的鸿沟，对第二个预测进行了首次直接测试。使用检出率作为生存率的代表，我们的结果支持了迁徙鸿沟假说，杂种的存活率低于亲本。这一发现是青少年特有的（相对于成年人），表明对杂种的选择在生命早期更强。杂种存活率的降低并不是通过针对中间表型的选择或表型之间的负相互作用来解释的。需要将迁移的具体特征联系起来的额外工作，但这些模式为迁移作为物种形成的生态驱动因素提供了强有力的支持。