Integrating flight mechanics, energetics and migration ecology in vertebrates

Animal locomotion is constrained by Newtonian laws of motion and therefore biomechanics is a useful approach for quantitative analysis of force and power requirements. Aerial locomotion in vertebrates is no exception, and arguably the most significant developments are to be found in this journal. Evolutionary birds and bats are very successful groups, doubtless largely because of their ability to shift location in a short time. This has enabled birds and to a lesser extent bats to perform seasonal long-distance migrations between habitats suitable for reproduction and survival. Power required to fly and potential flight range in relation to fuel load are two fundamental relationships derived from flight mechanics, which both serve as a foundation for the development of optimal migration theory. From this framework where biomechanics, energetics and ecology combine, we can analyse which of the alternative strategies migrants adopt. Such adaptive behaviours include the selection of optimal flight speed and the migratory travel itinerary. However, despite decades of research efforts, there are still many unsolved problems concerning flight mechanics and energetics of vertebrate flight. One such is how the power-speed relationship maps onto metabolic rate during flight, the so-called energy conversion efficiency. There is conflicting empirical evidence concerning how energy conversion possibly varies with flight speed, body mass and body size. As ultimately it is the metabolic energy consumption that is under selection pressure, this is an urgent question for the utility of flight mechanical principles in ecology. In this Review, I discuss this and other knowledge gaps in vertebrate flight and migration.