Ecological and behavioural constraints on range expansion in migratory birds

对于许多物种来说，不断变化的气候条件正在改变它们目前栖息的景观的适宜性。这些物种未来的成功可能取决于它们将自己的分布转移到条件合适的地区的能力，这将需要个体分散到新的地点。考虑到候鸟飞行的距离很远，可能预计它们最有能力进行这种扩散。然而，个别鸟类通常高度忠于它们的繁殖和冬季地点，因此扩散事件可能很少见。扩散活动的成功还将取决于可供散布的个体使用的栖息地的质量。因此，个体的扩散行为和栖息地的可获得性可能有助于或阻碍物种改变分布以应对气候变化的能力。探索这些生态和行为限制对候鸟的相对重要性需要一个系统，其中1.确定栖息地质量，2.记录活动范围大小和分布的变化，3.可以跟踪个体的扩散决定。近几十年来，冰岛黑尾信天翁的数量和活动范围一直在增加，并在夏季和冬季扩展到质量较差的栖息地。我们对这些神童的长期标记和跟踪表明，新繁殖地的鸟类往往也会使用新的冬季栖息地，而且它们往往比传统繁殖地的鸟类更大。更大的鸟可能在新的地点更常见，要么是因为它们能更好地适应这些地点的条件，要么是因为这些不太成功的个体更有可能分散去寻找配偶。因此，这项研究提供了一个独特的机会，通过测量新地点的质量和新地点鸟类成功繁殖的可能性，来探索迁徙物种范围扩大的后果。这项研究将极大地提高我们对迁徙物种扩大范围的潜力，以及这种范围转移的人口和进化后果的理解。

For many species, changing climatic conditions are altering the suitability of the landscapes that they currently inhabit. The future success of such species may depend on their ability to shift their distribution to areas where conditions are suitable, which will require individuals to disperse to new sites. Migratory birds might be expected to be most capable of such dispersal, given the vast distances over which they travel. However, individual birds are typically highly site-faithful to their breeding and winter sites, and so dispersal events can be rare. The success of dispersal events will also depend on the quality of the habitat available to dispersing individuals. Individual dispersal behaviour and habitat availability can therefore help or hinder a species' capacity to shift distribution in response to climate change. Exploring the relative importance of these ecological and behavioural constraints for migratory birds requires a system in which 1. habitat quality has been identified, 2. changes in range size and distribution have been documented and 3. individual dispersal decisions can be tracked. Icelandic black-tailed godwits have been increasing in number and range in recent decades and expanding into poorer quality habitats in summer and winter. Our long-term marking and tracking of these godwits has shown that birds in new breeding sites tend to also use new winter sites, and that they tend to be larger than birds in traditional breeding sites. Larger birds may be more common on new sites either because they can cope better with conditions in these sites or because these are less successful individuals that are more likely to disperse to try and find a mate. This study therefore provides a unique opportunity to explore the consequences of range expansion in a migratory species, by measuring the quality of new sites and the probability of birds in new sites breeding successfully. This study will greatly improve our understanding of the potential for migratory species to expand their ranges, and of the demographic and evolutionary consequences of such range shifts.