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 a re less severe than in the breeding area, or where the food supply is bette r, but the journey may be danger ous and its energy cost May be high. Simpl e conditions are presented for predicting whether migration will increase i ts probability of survival, or improve its energy balance. Speeds and energ y costs of migration are estimated for walking, swimming and flying vertebr ates of masses ranging from three grammes to three tonnes. Consideration of these in the light of the proposed conditions indicates that 1000 km (roun d trip) migrations may benefit flying vertebrates of all sizes, but are unl ikely 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 occur rence of long migrations among vertebrates.