Regulated changes in body mass and muscle mass in molting Blue-winged Teal for an early return to flight

Abstract

During the breeding season, Blue-winged Teal (Anas discors) undergo cyclic changes in body mass (M b ) and pectoral- and leg-muscle mass coincident with the simultaneous molt of all flight feathers. These conformational changes cannot be attributed to nutritional or metabolic demands, nor can they be a use-disuse phenomenon. A reduction of >>18% in body mass from premolt values produces wing loadings (M b /wing area) nearly equal to those seen premolt, allowing these birds to regain flight capability, although the flight feathers are <<75% of premolt length and area. A reduction of >>30% in flight-muscle mass represents about half of the total reduction in M b ; however, the lowest power necessary for flight, calculated for five different periods during the breeding season, is found at 75% of feather regrowth. Reduction in M b coupled with a >>40% increase in leg-muscle mass during the flightless period should permit these birds to achieve higher swimming speeds. The temporal and mechanical relationships of these conformation adjustments suggest that they are programmed or regulated to (i) permit the earliest possible return to flight after the molt-related flightless period and (ii) provide for faster predator-avoidance speeds across or under the water during the flightless period.