Maximal horizontal flight performance of hummingbirds: Effects of body mass and molt

Hovering and fast forward napping represent two strenuous types of flight t hat differ in aerodynamic power requirement. Maximal capabilities of ruby-t hroated hummingbirds (Archilochus colubris) in hovering and forward flight were compared under varying body mass and wing area. The capability to hove r in low-density gas mixtures was adversely affected by body mass, whereas the capability to fly in a wind tunnel did not show any adverse mass effect . Molting birds that lost primary flight feathers and reduced wing area als o displayed mass loss and loss of aerodynamic power and flight speed. This suggests that maximal flight speed is insensitive to short-term perturbatio ns of body mass but that molting is stressful and reduces the birds' speed and capacity for chase and escape. Hummingbirds' flight behavior in confine d space was also investigated. Birds reduced their speeds flying through a narrow tube to approximately one-fifth of that in the wind tunnel and did n ot display differences under varying body mass and wing area. Hence, perfor mance in the night tube was submaximal and did not correlate with performan ce variation in the wind tunnel. For ruby-throated hummingbirds, both maxim al mass-specific aerodynamic Fewer derived from hovering performance in low -density air media and maximal flight velocity measured in the wind tunnel were invariant with body mass.