Data from: Low migratory connectivity is common in long-distance migrant birds

1. Estimating how much long-distance migrant populations spread out and
mix during the non-breeding season (migratory connectivity) is essential
for understanding and predicting population dynamics in the face of global
change. 2. We quantify variation in population spread and inter-population
mixing in long-distance, terrestrial migrant land-bird populations (712
individuals from 98 populations of 45 species, from tagging studies in the
Neotropic and Afro-Palearctic flyways). We evaluate the Mantel test as a
metric of migratory connectivity, and explore the extent to which variance
in population spread can be explained simply by geography. 3. The mean
distance between two individuals from the same population during the
non-breeding season was 743 km, covering 10–20% of the maximum width of
Africa / South America. Individuals from different breeding populations
tended to mix during the non-breeding season, though spatial segregation
was maintained in species with relatively large non-breeding ranges (and,
to a lesser extent, those with low population-level spread). A substantial
amount of between-population variation in population spread was predicted
simply by geography, with populations using non-breeding zones with
limited land availability (e.g. Central America compared to South America)
showing lower population spread. 4. The high levels of population spread
suggest that deterministic migration strategies are not generally
adaptive; this makes sense in the context of the recent evolution of the
systems, and the spatial and temporal unpredictability of non-breeding
habitat. 5. The conservation implications of generally low connectivity
are that the loss (or protection) of any non-breeding site will have a
diffuse but widespread effect on many breeding populations. Although low
connectivity should engender population resilience to shifts in habitat
(e.g. due to climate change), we suggest it may increase susceptibility to
habitat loss. We hypothesise that because a migrant species cannot adapt
to both simultaneously, migrants generally may be more susceptible to
population declines in the face of concurrent anthropogenic habitat and
climate change.

1. 评估长距离迁徙种群在非繁殖季节的扩散与混合程度（即迁徙连通性，migratory connectivity）对于理解和预测全球变化背景下的种群动态至关重要。
2. 我们量化了陆地长距离迁徙鸟类种群的扩散变异及种群间混合程度（数据来源于新热带区和非洲-古北界迁飞路线的标记研究，涵盖45个物种、98个种群的712只个体）。我们将曼特尔检验（Mantel test）作为迁徙连通性的度量指标进行评估，并探究种群扩散的变异在多大程度上可仅通过地理因素解释。
3. 同一种群个体在非繁殖季节的平均距离为743公里，约占非洲或南美洲最大宽度的10%-20%。不同繁殖种群的个体在非繁殖季节倾向于混合，但非繁殖范围相对较大的物种（以及在较小程度上，种群水平扩散较低的物种）仍保持空间隔离。种群扩散的种群间变异中，相当一部分可仅通过地理因素预测：使用陆地资源有限的非繁殖区域（如相较于南美洲的中美洲）的种群，其扩散程度更低。
4. 高种群扩散水平表明，确定性迁徙策略通常不具备适应性；这一现象在系统近期演化及非繁殖栖息地时空不可预测性的背景下具有合理性。
5. 普遍较低的连通性带来的保护意义在于：任何非繁殖地的丧失（或保护）都会对众多繁殖种群产生扩散但广泛的影响。尽管低连通性应能增强种群对栖息地变化（如气候变化所致）的恢复力，但我们认为它可能增加种群对栖息地丧失的敏感性。我们假设，由于迁徙物种无法同时适应这两种变化，在人为栖息地变化与气候变化同时发生的情况下，迁徙物种总体上可能更易出现种群数量下降。