Airspeed adjustment and lipid reserves in migratory Neotropical butterflies

1. Aerodynamic theory predicts that migrant fliers should reduce their speed of flight as endogenous energy reserves are gradually consumed. This prediction was tested for butterfly species (Pieridae and Nymphalidae) that engage in annual rainy season migrations through central Panama. 2. Direct airspeed measurements were made on butterflies in natural free flight, followed by chloroform : methanol extractions of abdominal lipids from the same insects. 3. Among individuals within particular species/gender subsets, airspeeds during flight were higher with greater lipid content following adjustment for body mass. Although it was not possible to measure lipid content repeatedly on a single insect, these comparisons among individuals for five migratory species suggest that butterflies reduce their flight speed as lipid reserves are progressively depleted. 4. Because choice of airspeed can strongly influence the rate of energetic expenditure, these results together with previously described strategies of wind drift compensation in the same taxa demonstrate sophisticated long-distance orientation and optimization strategies by migratory Neotropical butterflies flying within the boundary layer.