Flight, especially taking off and landing, requires a huge amount of energy-more than humans need even for running. Taking flight is less demanding for small birds than it is for large ones, but small birds need more energy to stay warm. In keeping with their enormous energy needs, birds have an extremely fast metabolism, which includes the chemical reactions involved in releasing stored energy from food. The high body temperature of birds-40° to 42° C (104° to about 108° F)-provides an environment that supports rapid chemical reactions.

To sustain this high-speed metabolism, birds need an abundant supply of oxygen, which combines with food molecules within cells to release energy. The respiratory, or breathing, system of birds is adapted to meet their special needs. Unlike humans, birds have lungs with an opening at each end. New air enters the lungs from one end, and used air goes out the other end. The lungs are connected to a series of air sacs, which facilitate the movement of air. Birds breathe faster than any other animal. For example, a flying pigeon breathes 450 times each minute, whereas a human, when running, might breathe only about 30 times each minute.