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

在整个欧洲，据报道，许多候鸟物种的种群数量正在广泛而迅速地下降。例如，在过去的15年里，英国繁殖的杜鹃、夜莺和斑点飞鸟的数量减少了一半。这些下降的原因尚不清楚，但对长途(特别是撒哈拉以南)迁徙物种的更大影响已导致人们认为，它们是由越冬地点的变化推动的。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、迁徙或越冬地点的环境变化都可能是导致种群数量下降的原因。事实上，在英国，我们之前已经表明，并不是所有的移民人口都在下降，而且常住人口和移民人口在同一地区的表现都更好。这表明，繁殖地的变化，如农业集约化导致栖息地可获得性的下降，可能是(居民和迁徙的)人口趋势的主要驱动因素，但迁徙路线或冬季地点条件的变化可能会导致额外的“迁徙成本”。由于欧洲和撒哈拉以南非洲可用于保护行动的资源和基础设施差异很大，在开始实施可能难以实施的代价高昂的行动之前，确定资源最好花在哪里是极其重要的。人们对迁徙到撒哈拉以南非洲的候鸟物种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非常好的监测，这要归功于欧洲各地开展的基于志愿者的鸟类调查，以提供对许多物种每年丰度的国家估计。此外，在许多国家，以志愿者为基础的持续努力的鸟鸣被用来监测存活和招募到繁殖种群中。这些数据来源在整个欧洲范围内的规模为解决这些问题提供了一个强有力的机会，方法是对(A)在欧洲各地繁殖但迁徙路线不同的种群进行种内比较，以及(B)居留物种和迁徙物种之间这些模式的一致性。因此，我们建议首次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，确定人口下降最严重的地点和栖息地，并量化推动人口下降的人口和环境因素。量化繁殖和非繁殖季节过程对这些物种减少的相对贡献，将是确定对这些物种采取有针对性的保护行动的最合适地点的关键，并将为复杂迁徙系统中特定物种的研究提供框架。

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.