Wind Drift Compensation in Migrating Dragonfiles

Tailwind drift compensation serves to maximize a migrant’s flight distance on a given amount of energy, and crosswind drift compensation serves to hold a course true and minimize the distance flown. With full or part com-pensation, airspeeds are predicted to increase with greater crosswind drift. To test whether migrating dragonflies compensated for wind drift, I measured the velocity and heading of Pantala hymenaea and P. flavescens in natural flight over a lake and the ambient wind speed and direction. P. hymenaea flew north-easterly (58◦), whereas P. flavescens flew significantly more north–north easterly (74◦) throughout the day. Pantala spp. demonstrated part compensa-tion for changes in crosswind drift within individuals (mean compensation = 54%, P = 0.0000), evidence for use of a ground reference to correct for drift when flying over water. Among individuals, P. flayescens compensated for crosswind drift. P. hymenaea overcompensated and then drifted downwind on one morning and compensated for crosswind drift on the next. As pre-dicted from optimal migration theory, airspeed (5.0 m/s for both species with no tailwind) decreased with tailwind velocity both among individuals (data for both species pooled [n = 19], P < 0.0001) and within each individual as it crossed the lake (P = 0.0016). KEY WORDS: insect; flight; energetics; optimal migration; behavioral ecology; tropical