Migratory populations in a changing world: connectivity, range shifts, and responses

As the natural world undergoes rapid change, there are particularly strong impacts on migratory species. Their reliance on multiple distinct ranges complicates their spatial population dynamics in ways we are yet to fully understand. In this thesis I explore aspects of spatial population dynamics unique to migrants, focussing on dispersal and its significance for connectivity, range shifts, and population growth within a changing world. I firstly explore the use of quantitative metrics (Mantel correlations) of migratory connectivity strength, highlighting their vulnerability to bias and scale-dependence. Through simulations, I provide guidelines for reducing biases when applying such metrics. I then demonstrate that migratory connectivity is an emergent pattern driven by full-cycle dispersal dynamics, leading to a novel theoretical framework for dispersal in migrants including ‘non-reproductive dispersal’ – movements between consecutively-occupied non-breeding sites. This previously-undefined form of dispersal is driven by changes in migratory programmes, and is likely to be vital for species shifting their seasonal ranges. I explore this empirically by examining the role of cultural inheritance of migratory programmes in explaining between-species variation in range shift rates of North-American migratory birds. I find that species that migrate in mixed-age flocks (and thus have a high capacity for cultural inheritance) showed faster rates of non-breeding range shift than solo migrants, after controlling for phylogeny and other species traits. This supports the hypothesis that mechanisms of migratory inheritance play a key role in determining spatial dynamics of migratory populations. Finally, I assessed how climatic conditions experienced during distinct seasonal stages influence year-to-year population growth rates across North-American migrants, finding that season-specific climates had relatively little power in explaining interannual changes in population size. Overall, this thesis demonstrates the importance of several aspects of spatial dynamics unique to migrants, providing promising avenues for improving our understanding of migratory ecology in a changing world