The biomechanical foundations of vertebrate migration: linking theory, energetics and ecology

Anders Hedenström

Lund University, Sweden

Optimal migration theory rests on two fundamental biomechanical relationships derived from aerodynamic theory: (1) the power P required to fly in relation to airspeed U, P(U), and (2) the potential flight range Y in relation to the fuel load f, Y(f ). The former typically shows a U-shaped function, while the latter is a function of diminishing return. Since both relationships are curvilinear they are amenable for optimization theory if combined with relevant currency assumptions and optimization criteria. From the P(U)-curve we derive predictions about flight behaviors, including the selection of optimal flight speeds, optimal climb rate, and limits to fuel-carrying capacity. Measurements of flight speeds often show qualitative agreement with theory, while quantitative differences can probably be attributed to limitations of the relatively simple flight mechanical models. The mechanical P(U)-curve has a metabolic counterpart related through the energy conversion efficiency. Early wind tunnel studies contributed to the assumption that energy conversion efficiency is a universal invariant. However, more recent studies suggest that it varies with both body size and flight effort. The flight range equation is fundamental to the analysis of migration strategies and the geometry of migration routes. Depending on whether migrants are selected to minimize energy, time or safety of migration alternative fueling and flight behaviors emerge. With the recent development of small multi sensor data loggers detailed characteristics of entire annual migrations can be obtained, allowing us to evaluate which of alternative migration strategies birds and bats follow. Here, I review the current theoretical and empirical state regarding flight and migration strategies in aerial vertebrates.