What is migration?

We outline a general scheme for migration that applies across taxa, incorporates the several varieties of migration, and includes all levels of biological organization, from genes to populations. The scheme links the environment, pathways, traits, and genes, and highlights the selective forces that shape and maintain migratory adaptation. We endorse an individual-based behavioral definition of migration that allows an objective distinction between migration and other forms of movement. We recognize migration as an adaptation to resources that fluctuate spatiotemporally either seasonally or less predictably, and note that it is often preemptive. Migration plays a central role in the spatial dynamics of mobile populations, and is largely distinct in both form and function from the within-population mixing arising from postnatal dispersal and from the interpatch movements characteristic of metapopulations. We call for more interaction between biologists studying different taxa and different forms of movement, and between behaviorists and population ecologists.