Soaring flight of monarch butterflies, Danaus plexippus (Lepidoptera: Danaidae), during the late summer migration in southern Ontario

Adult Danaus plexippus in southern Ontario frequently engage in soaring flight during their late summer migration. They utilize both ascending air currents (lift) produced by winds blowing up slopes and thermals (bubbles or columns of air that are rising because they are warmer and lighter than the surrounding air). The butterflies appear to be very efficient and exhibit different flying techniques when encountering various types of lift. When the weather is favourable, soaring is the main mode of flight and the butterflies achieve heights above the ground of at least 300 m. Extended soaring flight in thermals was always associated with tail winds.A sample of 18 specimens gave an average mass of 566 ± 81 mg and an average wing loading of 0.018 ± 0.002 g/cm2 (1.77 N/m2). Gliding performance parameters were measured for two specimens ballasted to 450 mg and the results were extrapolated to 600-mg individuals. The energetic advantages of utilizing soaring flight in terms of extension of maximum range is disc...