The heaviest flying birds are the kori bustard and the great bustard, which can both weigh up to 42 pounds each. The males of each species are the largest, with females weighing far less.
The kori bustard, or Ardeotis Koriis, is typically native to Africa, while the great bustard, or Otis tarda, is native to Europe and Asia. In spite of their weight, great bustards still migrate nearly 2,500 miles each year. While most long-distance migratory birds glide to conserve their energy, great bustards flap the entire distance, taking frequent breaks to conserve energy. One study found that great bustards take around four months to complete their migration, but spend only 2 to 6 percent of that time airborne.