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

Abstract

One prediction derived from optimal migration theory is that migrating animals that maximize their flight distance on a given amount of energy will decrease 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 in the morning and then turned southeasterly to head downwind in the afternoon. Changes in heading and track directions of P. sennae were not related to changes in the position of the solar azimuth. As predicted from optimal migration theory, flight velocities of females decreased in a tailwind to minimize energy consumption. However, males did not show any compensation for tailwinds. Females are minimizing energy consumption, whereas males may be minimizing the time to reach the destination site in order to maximize matings with newly arrived or newly emerged females. Orientation of females changed before that of males, presumably because their greater reproductive load imposed greater flight costs and limited flight fuels.