Swarming and mass migration are spectacular and sometimes devastating features of the biology of various animal species. These phenomena are typically associated with actual or anticipated depletion of food resources after an increase in population density, but the mechanisms driving such collective movements are poorly understood. Here we reveal that insects in large, coordinated migratory bands consisting of millions of Mormon crickets in western North America were deprived of two essential nutritional resources: protein and salt. The insects themselves provided a major source of these nutrients, and cannibalism was rife. We show that protein and salt satiation reduced cannibalism and that protein satiation inhibited walking. Additionally, experimentally reducing the motility or mobility of crickets substantially increased their risk of being cannibalized by other band members. As a result, the availability of protein and salt in the habitat will influence the extent to which bands march, both through the direct effect of nutrient state on locomotion and indirectly through the threat of cannibalism by resource-deprived crickets approaching from the rear. The crickets are, in effect, on a forced march. Migratory band formation and subsequent mass movement, therefore, are manifestations of specific tradeoffs between the costs and benefits of group living. Bands afford antipredator benefits to individual group members. Group movement then mitigates the resulting costs of intraspecific competition, namely local depletion of nutritional resources and the associated increased risk of cannibalism.