We collected data for flight speeds of foraging common nighthawks (Chordeiles minor) flying under natural conditions using a Doppler radar and compared them with data collected by timing flights over a known distance for the same species. We also compared our Doppler radar measures of flight speed with the values of minimum power speed (V_mp) and maximum range speed (V_mr) predicted from aerodynamic models. Our measurements showed that under natural conditions, nighthawks flew at 6.5 ± 1.1 m-s⁻¹, significantly faster than the 5.3 ± 1.2 m-s⁻¹ derived by timing flights over measured distances. Doppler measurements indicate that on average, nighthawks flew significantly faster than the predicted V_mp, but slower than the predicted V_mr although there was considerable variation. Our results confirm that the method of measuring flight speeds can be important and suggest that the mechanics of hunting insects and not energetics, may be most important in determining the flight speed used by foraging common nighthawks.