The ecology and evolution of partial migration

Partial migration, where populations of animals are composed of a mixture of resident and migratory individuals, is a widespread phenomenon in nature. It has been reported to occur in all major vertebrate groups, and can have significant ecological consequences. Here we give an overview of the ecology and evolution of partial migration in animals. We firstly review the different types of partial migration, and assess the ecological drivers responsible for driving individual differences in migratory tendency within populations. A variety of factors can be important in promoting the evolution of partial migration, including competition for resources or breeding opportunities, predation risk and intraspecific niche diversity. Often various factors act synergistically to create complex patterns of movement polymorphism within populations. The question of how partial migration is maintained over evolutionary timescales is also addressed. Whilst many theoretical considerations of partial migration utilise an evolutionary stable state (ESS) paradigm, empirical evidence for this is lacking. Rather the evidence suggests that partial migration is mostly condition dependent, and the optimum outcome for an individual is dependent upon its phenotype. What determines whether an individual follows a migratory or resident strategy is discussed in light of new theory and empirical data which supports the idea that environmentally responsive genetic thresholds are important across a range of species, from birds to fish, in proximately shaping migratory tendency. Finally we espouse our vision of how partial migration research will develop in the future, and suggest a number of exciting directions that studies into migratory dimorphism may take in the coming years.