HUMMINGBIRD HOVERING ENERGETICS DURING MOLT OF PRIMARY FLIGHT FEATHERS

How does a hovering hummingbird compensate for the loss of flight feat hers during moult when the mechanism of lift force generation by flapp ing wings is impaired? The flight performance of five individual ruby- throated hummingbirds with moulting primary flight feathers and reduce d wing area was compared with that before their moult, Hummingbirds we re flown in reduced air densities using normoxic heliox so that a rang e of flight energetics was displayed, The rate of moulting and the ext ent of wing area loss varied among individuals, One female could toler ate a 30% loss of wing area in moulting and hew with only three outer primaries per wing, Further exploratory study using the artificial red uction of wing area, either by cutting the tips of the outer primaries of a male or by plucking the secondaries of two females, suggested th at secondaries play a minor role in lift force generation during hover ing whereas the tip area of primaries is crucial, For the five birds, ranges of whole-bird oxygen consumption rates, wingbeat kinematics (st roke amplitude) and lift coefficients did not vary during the moult, T his constancy was mainly achieved through weight loss that alleviated aerodynamic force requirements for weight support during hovering, Sin ce the metabolic power expenditure during moult was similar to that of normal birds but the mechanical power requirement was reduced, the fl ight efficiency also showed a sharp reduction during moult, This incre ased cost of flight may result from disruption of the integrity of the flight machinery, Overall, the control of body mass in hummingbirds c an provide similar aerodynamic, muscle mechanical and physiological ca pacities under conditions of variable flight demand.