The concept of 'energy height', developed by Pennycuick (2003), measures a bird's fuel reserves in terms of what the bird can do with the fuel, rather than its energy content. For a bird that travels by flapping flight, the fuel energy height is defined as the height which a bird would reach if all of its stored fuel energy were converted into work by the flight muscles, and used to lift the bird against gravity. A soaring bird, starting from a given height above ground, can glide a certain distance in exchange for using up its initial store of potential energy. Likewise, a store of energy fuel in the form of fat corresponds to a virtual energy height from which the bird comes down at a virtual angle that depends on its aerodynamic efficiency. This virtual angle is much the same as the gliding angle. The notion of energy height thus provides a way to compare species that migrate by powered flight, replenishing their fuel energy height at stopovers, with those that migrate by soaring, replenishing their energy height by being lifted by air currents. Rates of climb in thermals are typically higher than the rates achieved through powered flapping flight, but the available height band for flight in thermals is at least one order of magnitude smaller, and the intervals at which energy replenishment is needed are correspondingly shorter (Pennycuick 2003). Albatrosses replenish their kinetic energy by exploiting discontinuities in wind flow and wave action, requiring replenishment at intervals of tens of seconds, a further two orders of magnitude shorter than in thermal soaring. In such a system, however, a bird could in theory migrate over long distances with little more daily energy expenditure than it would use at other times, a prediction borne out by the study of albatrosses mentioned above (Weimerskirch et al. 2000).
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