Overall migration speed is likely to be a trait under selection pressure in animals migrating annually between habitats for reproduction and survival. A general expression was used for migration speed (Vmigr), accounting for energy accumulation and transport, and derive how Vmigr scales with body mass for three types of migratory locomotion: running, swimming and flying (powered and soaring flight). Migration speed is predicted to increase with increasing body mass in animals that run (∝m1/11), swim (∝m1/24) and fly by soaring (∝m0.22), whereas in animals migrating by powered flight it decreases (∝m−1/4). How the relative duration of fuel accumulation to that of migratory locomotion scales with body mass is also derived. This proportion should increase in animals that run and fly, but should remain unaffected by body size in swimmers. The suggestion is made that in runners and swimmers, selection for enhanced migration speed could provide an explanation to Cope's rule, i.e. the observation that body size gradually tends to increase over time.