The Drag Paradox: Measurements of Flight Performance and Body Drag in Flying Birds

Abstract

Central to understanding the achievements of migrating birds and the constraints within which they operate is the ability to estimate the flight performance that a migrant can maintain for sustained periods. To date, there have been relatively few direct measurements of avian flight energetics, and because of the complexity of the techniques involved, few of the available data have been subject to appropriate controls. Attention has therefore focussed on the use of analytical models based on aerodynamic theory to estimate measures of flight performance such as flight speed, power and range, and which can be used to predict flight behaviour under a range of circumstances. Application of such models has been very effective in developing an understanding of, for instance, the factors that determine the choice of flight speed, flight strategies in response to varying winds and/or topography and feeding strategies prior to or during extended migration (Alerstam and Lind-ström 1990; Berthold 2001; Alerstam 2002). The most common currencies used in formulating these questions are energy and time, both of which may have been the target of significant selective pressures on migrating birds. Accordingly, the most affective aerodynamic models are those that are able to make realistic predictions of flight speed and power in a range of different birds.