Advances in Animal Flight Aerodynamics Through Flow Measurement

Animal flight is a fruitful area of study for biologists because the abundance of extant and extinct forms radiating from a small handful of unrelated ancestral states provides such a rich resource filled with examples of variation and convergence. Uniting the disparate perspectives of engineering and classical fields of biology has given rise to an integrated biomechanical analysis aimed at understanding the physical basis governing animal morphology and locomotion. Over the past 8 years, Particle Image Velocimetry (PIV) has revolutionised our ability to analyse bird (Spedding et al. 2003b), insect (Bomphrey et al. 2005) and bat (Hedenström et al. 2007) flight performance by quantifying the flow fields left in wakes as they fly in wind tunnels. In doing so, existing models have been variously rejected and supported (Spedding et al. 2003b), and paradoxes resolved (Warrick et al. 2009). Nevertheless, PIV remains a time-consuming enterprise, and the vast number of flying animal species available for study leads to the familiar dilemma of whether to focus on model species or take a comparative approach. The latter is necessarily lessdetailed, but would enable us to gauge morphological and kinematic trends that persist in guilds of fliers with defined ecological strategies. On the other hand, the model species approach might select target animals on the basis of extremes in performance, and in doing so better characterize the mechanisms of specialists. Examples of such peculiarities are abundant: the insectivorous common swift leads an almost entirely airborne existence (Lack 1956), hummingbirds manage to generate sufficient lift to hover even at extreme altitudes (Altshuler and Dudley 2003), large bats can support body weights of more than a kilogram (Kunz and Jones 2000), dragonflies have remarkable prey capture rates made possible by outstanding aerobatics (97%; Olberg et al. 2000), and honey bees frequently airlift pollen loads up to 80% of their own bodyweight (Winston 1987). Model species approaches are undoubtedly illuminating from an ecological perspective, but oftentimes do not ascertain which, if any, of the morphological adaptations are responsible for the performance extrema under investigation, nor the putative mechanism possessed by the guild’s members upon which natural selection operates to increase fitness. Nevertheless phenotypes pertaining to aerial locomotor biomechanics are measurable, and can fit within a framework to explain trends in kinematics, power requirements, range, and behaviour, and furthermore determine how these variables are constrained by the physical world.