E12 Metabolism and available power

Topic

A bird's energy system describes how it converts its biological fuel reserves—fat, glucose, and glycogen—into the mechanical power available for flight. It is the module that links the bird's biology—its metabolism, respiration, and nutrition—with the physics of its flight, expressed as the power it can sustain.

Without the bird's energy system, the modules describing wing-profile aerodynamics and wing-beat kinematics explain only what a bird could do in principle, in a world where energy never ran out. By incorporating the energy system, however, we can determine what the bird is actually capable of doing, given the energy reserves it carries and the rate at which its metabolism consumes them.

Fat is the fuel that sustains long-distance flight within this energy system, as it possesses the highest energy density of any known biological fuel: approximately thirty-seven kilojoules per gram—more than double the energy density of carbohydrates. Thanks to this high energy density, migratory birds can accumulate fat reserves equivalent to half their own body mass before embarking on a long migration.

The rate at which different birds consume these fat reserves during migration varies enormously by species: a hummingbird on a transoceanic migration consumes mere hundredths of a gram of fat per hour, whereas an Arctic goose loses tens of grams per hour during the non-stop three-thousand-kilometer journey across the Atlantic Ocean.

This same fat reserve—consumed at a rate determined by the species and the mechanical power required for the flight—determines the duration of a migratory flight before the bird's energy system reaches its limit: as long as the fat reserves last, the bird flies in its optimal performance phase; When that reserve begins to run low, the bird's energy system can no longer sustain—in practice—the same flight that the wing profile's aerodynamics and the kinematics of the wingbeat would, in principle, still allow.

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