Ecological and behavioural constraints on range expansion in migratory birds

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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.