When Is Migration Worthwhile for Animals That Walk, Swim or Fly?

An animal that migrates may travel to an area where the hazards of winter are less severe than in the breeding area, or where the food supply is better, but the journey may be dangerous and its energy cost may be high. Simple conditions are presented for predicting whether migration will increase its probability of survival, or improve its energy balance. Speeds and energy costs of migration are estimated for walking, swimming and flying vertebrates of masses ranging from three grammes to three tonnes. Consideration of these in the light of the proposed conditions indicates that 1000 km (round trip) migrations may benefit flying vertebrates of all sizes, but are unlikely to be beneficial for small swimmers or runners. Migrations of 10,000 km are expected to be beneficial only for fliers and large endothermic (but not ectothermic) swimmers. These conclusions are consistent with the occurrence of long migrations among vertebrates.