Hummingbird hovering energetics during moult of primary flight feathers.

How does a hovering hummingbird compensate for the loss of flight feathers during moult when the mechanism of lift force generation by flapping wings is impaired? The flight performance of five individual ruby-throated hummingbirds with moulting primary flight feathers and reduced wing area was compared with that before their moult. Hummingbirds were flown in reduced air densities using normoxic heliox so that a range of flight energetics was displayed. The rate of moulting and the extent of wing area loss varied among individuals. One female could tolerate a 30% loss of wing area in moulting and flew with only three outer primaries per wing. Further exploratory study using the artificial reduction of wing area, either by cutting the tips of the outer primaries of a male or by plucking the secondaries of two females, suggested that secondaries play a minor role in lift force generation during hovering whereas the tip area of primaries is crucial. For the five birds, ranges of whole-bird oxygen consumption rates, wingbeat kinematics (stroke amplitude) and lift coefficients did not vary during the moult. This constancy was mainly achieved through weight loss that alleviated aerodynamic force requirements for weight support during hovering. Since the metabolic power expenditure during moult was similar to that of normal birds but the mechanical power requirement was reduced, the flight efficiency also showed a sharp reduction during moult. This increased cost of flight may result from disruption of the integrity of the flight machinery. Overall, the control of body mass in hummingbirds can provide similar aerodynamic, muscle mechanical and physiological capacities under conditions of variable flight demand.