Sexual differences in tailwind drift compensation in Phoebis sennae butterflies (Lepidoptera : Pieridae) migrating over seas

One prediction derived from optimal migration theory is that migrating anim als that maximize their flight distance on a given amount of energy will de crease their airspeed in a tailwind and increase it in a headwind. To test this in a migrating butterfly, I followed male and female cloudless sulfur butterflies Phoebis sennae (Pieridae) migrating from Colombia toward Panama over the Caribbean Sea. P. sennae headed westerly over the Caribbean Sea i n the morning and then turned southeasterly to head downwind in the afterno on. Changes in heading and track directions of P. sennae were not related t o changes in the position of the solar azimuth. As predicted from optimal m igration theory, flight velocities of females decreased in a tailwind to mi nimize energy consumption. However, males did not show any compensation for tail-winds. Females are minimizing energy consumption, whereas males may b e minimizing the time to reach the destination site in order to maximize ma tings with newly arrived or newly emerged females. Orientation of females c hanged before that of males, presumably because their greater reproductive load imposed greater flight costs and limited flight fuels.