Flight speeds of migrating birds: a test of maximum range speed predictions from three aerodynamic equations

Many temperate birds invest considerable time and energy totravel the long distances between their breeding grounds andwintering areas. It has generally been assumed therefore thatto minimize the energy cost of migration (and thus maximizefuel economy) birds ought to fly at speeds that maximize thedistance travelled per unit of energy expended (termed the maximumrange speed, V_mr). I tested this idea by comparing literaturereports of flight speeds for 48 avian species on migration andcomparing them to predictions of V_mr derived from three aerodynamicequations (Tucker, Pennycuick, and Greenewalt). No single equationmade V_mr predictions that matched the full range of observedspeeds. Species weighing 0.3 kg–3 kg (Greenewalt equation)and 0.1 kg–1 kg (Pennycuick equation) generally migratedat V_mr, but this represents only 42% (20/48) and 40% (19/48)of the total number surveyed, respectively. Deviations fromV_mr outside these ranges varied systematically with mass. Lighterspecies almost always flew faster than V_mr, whereas heavierspecies showed the opposite trend. The latter group is likelyconstrained to fly below V_mr due to limits on metabolic performanceimposed by mass-specific scaling effects. The Tucker equationalmost always predicted V_mr values that were less than observedspeeds.