Reduced rainfall in tropical regions decreases the availability of food resources for overwintering songbirds, with negative impacts on their body condition. Increasingly dry conditions at tropical wintering sites as a result of climate change may impact the ability of temperate-breeding songbirds to prepare for and execute their spring migration. We studied the carry-over effects of natural climatic fluctuations created by the El Niño–Southern Oscillation (ENSO) in tropical overwintering areas on 7 long-distance migratory songbirds at a Canadian stopover site. We used the Normalized Difference Vegetation Index as a proxy for tropical habitat productivity and resource availability and a 34-year bird banding dataset from Long Point, Ontario, Canada to assess migration timing and stopover body condition. To increase our ability to detect potential carry-over effects, we employed a novel approach of using recent migratory connectivity studies to restrict the wintering ranges to areas most likely used by individuals passing through southern Ontario. Using linear models, we found that overwinter habitat productivity was significantly negatively influenced by dry ENSO events in the overwintering ranges in 3 of 7 species, with a fourth near-significant. Subregional differences in the effect of ENSO on precipitation patterns may explain why we did not detect a consistent effect of ENSO on overwinter habitat productivity for all species. Despite restricting the wintering range and using a robust dataset for species with diverse life histories, we detected only weak and often conflicting evidence of population-level carry-over effects from dry ENSO events and overwinter habitat productivity. Negative carry-over effects may be strongest and most evident during the earlier stages of migration because birds may be able to compensate to some extent for poor departure condition and late departure while en route.

中文翻译：

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越冬气候和栖息地生产力对安大略省北部中途停留地迁徙鸣禽春季通过的遗留影响的证据不足

热带地区降雨量减少会减少越冬鸣禽的食物资源，对其身体状况产生负面影响。由于气候变化，热带越冬地点日益干燥的条件可能会影响温带繁殖的鸣禽准备和执行春季迁徙的能力。我们研究了厄尔尼诺-南方涛动 (ENSO) 在热带越冬区对加拿大中途停留地点的 7 只长距离迁徙鸣禽产生的自然气候波动的遗留效应。我们使用归一化差异植被指数作为热带栖息地生产力和资源可用性的代表，并使用来自加拿大安大略省 Long Point 的 34 年鸟类条带数据集来评估迁徙时间和中途停留身体状况。为了提高我们检测潜在遗留效应的能力，我们采用了一种新方法，即利用最近的迁徙连通性研究，将越冬范围限制在最有可能被通过安大略省南部的个人使用的区域。使用线性模型，我们发现 7 个物种中有 3 个在越冬范围内的干 ENSO 事件对越冬栖息地生产力有显着负面影响，第四个接近显着。ENSO 对降水模式影响的次区域差异可以解释为什么我们没有检测到 ENSO 对所有物种的越冬栖息地生产力的一致影响。尽管限制了越冬范围并为具有不同生活史的物种使用了强大的数据集，我们只发现了干燥的ENSO事件和越冬栖息地生产力对种群水平的遗留影响的微弱且经常相互矛盾的证据。在迁徙的早期阶段，负面的遗留效应可能最为强烈和最明显，因为鸟类可能能够在一定程度上弥补在途中的不良离境条件和迟到的离境。