Seasonal interactions and rapid population declines in Afro-Palaearctic migratory birds

据报道，在整个欧洲，许多候鸟物种的数量正在广泛而迅速地下降。例如，在过去的15年里，英国的杜鹃、夜莺和斑捕蝇鸟的繁殖数量减少了一半。这些减少的原因尚不清楚，但对长途（特别是撒哈拉以南）迁徙物种的影响更大，导致人们认为它们是由越冬地点的变化驱动的。然而，了解迁徙物种的种群变化是复杂的，因为这些物种可以旅行的距离很远，而且它们在整个迁徙范围内依赖的地点很多。繁殖、迁徙或冬季地点的环境变化都可能导致这些种群的减少。事实上，在英国，我们之前已经表明，并非所有的移民人口都在下降，在同一地区，常住人口和移民人口的状况都有所改善。这表明，繁殖地的变化，如农业集约化导致的栖息地可用性下降，可能是人口趋势的主要驱动因素（在居民和迁徙者中），但迁徙路线或冬季地点条件的变化可能导致额外的“迁徙成本”。由于可用于保护行动的资源和基础设施在欧洲和撒哈拉以南非洲之间差别很大，因此，在采取可能难以实施的昂贵行动之前，确定资源的最佳用途是极其重要的。人们对迁徙到撒哈拉以南非洲的候鸟的冬季分布和栖息地利用知之甚少。然而，这些物种在繁殖季节得到了很好的监测，这要归功于在欧洲各地进行的以志愿者为基础的鸟类调查，这些调查提供了许多物种的年丰度的国家估计。此外，在许多国家，以志愿者为基础的不断努力的鸣鸟被用于监测生存和繁殖种群的招募。这些数据源的全欧洲范围内的规模提供了一个强有力的机会来解决这些问题，通过物种内比较(a)在欧洲各地繁殖但有不同迁徙路线的种群，以及(b)这些模式在常驻和迁徙物种之间的一致性。因此，我们首次提议整合这些大规模的长期数据，以探索整个欧洲人口下降的程度，确定下降最严重的地点和栖息地，并量化导致人口下降的人口和环境因素。量化繁殖期和非繁殖期过程对这些下降的相对贡献将是确定这些物种最适当的保护行动地点的关键，并将为复杂迁徙系统中物种特异性研究提供框架。

Across Europe, widespread and rapid population declines are currently being reported in many migratory bird species. For example, breeding populations of cuckoo, nightingale and spotted flycatcher in Britain have halved in the last 15 years. The causes of these declines are not understood, but the greater impact in long-distance (particularly sub-Saharan) migratory species has led to suggestions that they are being driven by changes on wintering sites. However, understanding population change in migratory species is complex because of the vast distances over which these species can travel, and the large number of locations on which they depend throughout their migratory ranges. Environmental changes in the breeding, migratory or winter locations could all be contributing to these population declines. Indeed, within the UK, we have previously shown that not all migrant populations are declining, and that both resident and migrant populations are faring better in the same areas. This suggests that changes on the breeding grounds, such as declines in habitat availability resulting from agricultural intensification, may be a primary driver of population trends (in both residents and migrants), but that changes in conditions on migration routes or winter locations may result in additional 'costs of being migrant'. As the resources and infrastructure available for conservation actions vary greatly between Europe and sub-Saharan Africa, it is extremely important to identify where resources may best be spent, before embarking on costly actions that may be difficult to implement.Very little is known about the winter distribution and habitat use of the migratory bird species that travel to sub-Saharan Africa. However, these species are very well-monitored during the breeding season, thanks to volunteer-based bird surveys that are carried out across Europe to provide national estimates of the annual abundance of many species. In addition, volunteer-based constant effort bird ringing in many countries is used to monitor survival and recruitment into breeding populations. The Europe-wide scale of these data sources provides a powerful opportunity to address these issues, through within-species comparisons of (a) populations that breed across Europe but have different migratory routes, and (b) the consistency of these patterns among resident and migratory species. We therefore propose to, for the first time, integrate these large-scale, long-term data, in order to explore the extent of the population declines across Europe, identify locations and habitats where declines are most severe, and quantify the demographic and environmental factors that are driving the population declines. Quantifying the relative contribution of breeding and non-breeding season processes to these declines will be key to identifying the most appropriate locations for targeted conservation actions for these species, and will provide the framework for species-specific research in complex migratory systems.