Physiological Challenges of Migration

Although there is little doubt that a number of avian species periodically fly over long distances, which has been defined as migration, how and why such mass movements evolved is still a matter of speculation. The range of body masses of birds that migrate ranges from very small individuals (hummingbirds) to quite large birds, such as geese. No matter the size of the bird, such sustained activity costs a great deal in the form of metabolic substrates. Data show that the principal fuel metabolized is lipids in the form of fatty acids. In addition, some birds will metabolize the protein of muscle and other organs when lipid stores are depleted. Carbohydrates (glucose) do not appear to be a substrate that is metabolized during sustained flight. Birds flying at extreme elevations show some adaptations of mitochondrial enzymes and muscle capillarity as compensation for low oxygen tension.