Marine radar is a tool widely used in the study of bird migration, but in most cases it cannot identify detected objects to species. For this reason airspeed is used as one of the main criteria to separate birdlike from insectlike targets, but this criterion has not been tested outside the temperate regions where it was developed. We used a theoretical approach for calculating minimum-power speeds (V_mp) and maximum-range speeds (V_mr) of birds and insects to estimate an interval of airspeeds from which an appropriate value of the criterion can be selected and to propose a criterion based on flight speed for the Isthmus of Tehuantepec in southern Mexico. This approach, with data from 36 species of insects and 60 of passerines either known to occur or similar to species occurring in southern Mexico, found that although V_mp of insects and birds overlaps at 5.5 m sec^-1, birds generally fly faster than insects. After combining our results with data from the literature, we conclude that the best airspeed to be used as criterion for distinguishing birdlike from insectlike targets lies within the range of 5.5–9.0 m sec^-1 and that 7 m sec ^-1 is an appropriate airspeed for our study area. These airspeeds are applicable in southern Mexico and in regions where birds and insects have physical dimensions similar to the species addressed in our study.