Data from: Non-breeding range size predicts the magnitude of population trends in trans-Saharan migratory passerine birds

Understanding why populations of some migratory species show a directional change over time, i.e. increase or decrease, while others do not, remains a challenge for ecological research. One possible explanation is that species with smaller non-breeding ranges may have more pronounced directional population trends, and their populations are thus more sensitive to the variation in environmental conditions in their non-breeding quarters. According to the serial residency hypothesis, this sensitivity should lead to higher magnitudes (i.e. absolute values) of population trends for species with smaller non-breeding ranges, with the direction of trend being either positive or negative depending on the nature of the environmental change. We tested this hypothesis using population trends over 2001–2012 for 36 sub-Saharan migratory passerine birds breeding in Europe. Namely, we related the magnitude of the species' population trends to the size of their sub-Saharan non-breeding grounds, whilst controlling for factors including number of migration routes, non-breeding habitat niche and wetness, breeding habitat type and life-history strategy. The magnitude of species’ population trends grew with decreasing absolute size of sub-Saharan non-breeding ranges, and this result remained significant when non-breeding range size was expressed relative to the size of the breeding range. After repeating the analysis with the trend direction, the relationship with the non-breeding range size disappeared, indicating that both population decreases and increases are frequent amongst species with small non-breeding range sizes. Therefore, species with small non-breeding ranges are at a higher risk of population decline due to adverse factors such as habitat loss or climatic extremes, but their populations are also more likely to increase when suitable conditions appear. As non-breeding ranges may originate from stochasticity of non-breeding site selection in naive birds (‘serial-residency’ hypothesis), it is crucial to maintain a network of stable and resilient habitats over large areas of birds’ non-breeding quarters.

阐明为何部分迁徙物种种群随时间呈现单向变化（即种群增长或下降），而其余物种种群却无此类变化，仍是生态学研究领域的未解难题。其中一种可能的解释为：非繁殖期活动范围（non-breeding range）较小的物种，其种群单向变化的趋势往往更为显著，因此这类种群对越冬地环境条件的波动更为敏感。依据连续居留假说（serial residency hypothesis），这种敏感性会使非繁殖期活动范围较小的物种种群的种群变化趋势幅度（即绝对值）更大，而趋势的正负方向则取决于环境变化的具体性质。
本研究以在欧洲繁殖的36种撒哈拉以南（sub-Saharan）迁徙雀形目鸟类（passerine birds）为研究对象，利用其2001至2012年的种群变化趋势数据对该假说进行了检验。具体而言，我们将物种种群变化趋势的幅度与其撒哈拉以南非繁殖地的面积进行相关性分析，同时控制了迁徙路线数量、非繁殖栖息地生态位（non-breeding habitat niche）与湿度、繁殖栖息地类型以及生活史策略（life-history strategy）等变量的影响。
研究结果显示，物种种群变化趋势的幅度随撒哈拉以南非繁殖期活动范围绝对面积的缩小而增大；且当以繁殖范围面积的相对值表示非繁殖期活动范围面积时，该结果依然显著。当针对种群变化趋势方向重复开展分析后，种群变化幅度与非繁殖期活动范围面积的相关关系不再显著，这表明在非繁殖期活动范围较小的物种中，种群下降与增长的情况均较为常见。
因此，非繁殖期活动范围较小的物种，因栖息地丧失或极端气候等不利因素而出现种群下降的风险更高；但当适宜环境条件出现时，其种群也更有可能实现增长。由于非繁殖期活动范围可能源于无经验鸟类的非繁殖地选择随机性（即‘连续居留假说’），因此在鸟类越冬地的广大区域内维持稳定且具有韧性的栖息地网络至关重要。