Abstract Nearly one in five bird species is migratory, but not all individuals within a migratory species necessarily migrate: in partially migratory species, some do and some do not. Such within-species variability provides a natural experiment for investigating the mechanisms driving bird migration. Previous studies at the species level suggest that migrating provides a way to escape harsh winters, and to secure an increased access to resources, particularly important during the breeding season. Urbanization, by altering local temperatures (‘heat island’ effect) and resource availability (e.g. through garbage or garden feeders) can buffer the effects of winter harshness and modify breeding-season resource availability, potentially affecting individual migratory strategies. Here, we use ringing data from twelve North American partially migratory bird species to investigate the effects of natural environmental conditions (winter temperature, breeding season resource surplus) and urbanization on the propensity of individuals to migrate. We find strong support for the hypothesis that individuals migrate to avoid harsh winters, with, for eleven species, significantly higher probabilities of residency in areas with milder winters. We also found (significant for five species) that resource surplus in the breeding season reduces the propensity to migrate. Finally, urbanization increased the likelihood that individuals remain year-round in their ranges, avoiding to migrate away from their breeding range (four species) or their wintering areas (eight species), after controlling for climate and resources. Our results thus indicate that bird migratory strategies will respond to global change – in climate and land use – and indeed are already doing so.

中文翻译：

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城市化对鸟类迁徙的影响

摘要 近五分之一的鸟类是迁徙的，但并非迁徙物种中的所有个体都必须迁徙：在部分迁徙的物种中，有些会，有些不会。这种种内变异为研究推动鸟类迁徙的机制提供了一个自然实验。先前在物种层面的研究表明，迁徙提供了一种逃避严冬的方式，并确保更多地获得资源，这在繁殖季节尤为重要。城市化通过改变当地温度（“热岛”效应）和资源可用性（例如通过垃圾或花园饲养者）可以缓冲冬季严寒的影响并改变繁殖季节的资源可用性，这可能会影响个体迁徙策略。这里，我们使用来自 12 种北美部分迁徙鸟类的振铃数据来研究自然环境条件（冬季温度、繁殖季节资源过剩）和城市化对个体迁徙倾向的影响。我们发现个体迁移以避免严冬的假设得到了强有力的支持，对于 11 种物种，在冬季较温和的地区居住的可能性显着更高。我们还发现（对五个物种意义重大）繁殖季节的资源过剩降低了迁徙的倾向。最后，城市化增加了个体在控制气候和资源后常年留在其栖息地，避免迁徙离开其繁殖地（四种）或越冬地区（八种）的可能性。