Towards an Optimal Strategy for Bird Flight Research

Abstract

Understanding bird flight and migration depends on field observations, theory and laboratory experiments. Like the legs of a tripod, these three elements depend on each other for support. No one leg can be extended too far beyond the others, without destabilising the structure. At the lowest level of theory is the physics of flight, which predicts the mechanical power required from the flight muscles. The purpose of the second level is to convert the primary power requirement of the flight muscles into a rate of fuel consumption, and to predict secondary power requirements from the heart and lungs, and basal metabolism. The third level, strategic theory, uses predictions from the lower levels to investigate migration strategies, which are supposedly optimal in the ecological and evolutionary contexts. A number of questions require attention to bridge existing gaps in the lower levels of theory, especially (1) the direct measurement of mechanical power output (not oxygen consumption), (2) the measurement of two biological constants, the isometric force exerted by one myosin filament, and the inverse power density of mitochondria, (3) the prediction of the mass and secondary power requirements of the heart and lungs, from the primary power requirement of the flight muscles, and (4) the nature of basal metabolism, and the manner in which it combines with other demands for power in the active animal. A complete theory at the physical and physiological levels can be used to construct computer simulations of migratory flight by particular birds. Such simulations can be used to approach the third (strategic) level of theory if values for input variables are obtained from field observations of real birds. The results can then be used to infer what the underlying strategy is, rather than postulating that the bird is selecting from a limited choice of possible strategies, derived analytically. The traditional emphasis on measurements of oxygen consumption, with little or no reference to the physics of the bird or the conditions of flight, does not help in building up a comprehensive body of theory, but this type of experiment could still be useful, to check the predictions of theory at the physical and physiological levels.