Birds have two independent locomotor systems: the forelimbs (wings) are used predominantly for aerial flight, but may be used for underwater propulsion, e.g. in penguins; the hindlimbs (legs) are used for running, surface swimming and diving. In birds of similar mass, energy consumption during flight is approximately 2.5 times greater than that when running or swimming at maximum speed. This difference is the result not only of the larger mass of the flight muscles compared with that of the leg muscles, but also of their greater oxidative capacity. Interestingly, the relationship of energy consumption to body mass in cursorial birds when running is similar to that of volant birds when flying. Energy consumption during diving may be as high in some birds (e.g. tufted duck) as when they are swimming at maximum sustainable speed, and this is not influenced by water temperature.
The composition of the flight and leg muscles is different. The muscles of the leg consist of deeply situated slow oxidative (SO) fibres, which are active during quiet standing and walking, fast oxidative glycolytic (FOG) fibres, which are recruited during walking and sustained running or swimming, and peripherally located fast glycolytic (FG) fibres, which are recruited at the highest running or swimming speeds. In most volant birds, the pectoralis muscle consists predominantly of FOG fibres with a smaller percentage of FG fibres. There is some controversy over the occurrence of SO fibres in some species, although they are most probably present in those that glide. The FOG fibres are highly oxidative, with a high capillary density.
The respiratory and cardiovascular adjustments that occur during flying, running and diving are described, and the ability of some species of birds to fly at extremely high altitudes, where the partial pressure is one-third of the sea level value, is discussed.