There is great diversity in the animal species that migrate, the biomechanics that propel their locomotion and the ecosystems through which they transit. This diversity, however, is unified by a common condition: the relative suitability of places changes in predictable and cyclical ways. Owing to the periodicity of environmental change (e.g., seasons) and animal life-cycles (e.g., growth and maturation) locations become favorable, lose favorability and become favorable again at somewhat regular intervals. Migratory animals have adapted to these predictable fluctuations by moving among locations, sometimes over extraordinary distances (Figure 1).