Body Composition of Captive White-Crowned Sparrows during Postnuptial Molt

A five-compartment analysis of body composition was performed on indoor and outdoor captive White-crowned Sparrows before, during, and after their postnuptial molt. Body weight was monitored simultaneously in four experimental groups. The duration of postnuptial molt in indoor and outdoor birds averaged 60 days. The body weight of outdoor captives in Pullman changed similarly to that seen during the early stages of molt in Fairbanks captives, although late molt autumnal fattening is less intense and more irregular in Pullman captives. Birds on a constant photoperiod at 5° and 15°C exhibited early weight gains, but these gains were either unsustained or converted to weight losses as molt progressed. Total body water in lean birds averaged 70% during molt and 68% after molt. The absolute weight of body water as well as body weight increased during molt. Thus the fractional TBW was essentially unchanged. The stability of the plumage-free lean body mass during molt reflected the negligible extent to which body protein is catabolized, although nocturnal protein degradation was not examined. The lean body weight during molt averaged 23.8% of the lean wet weight, plumage free, or about 5.6 g, but both these figures were only slightly higher after the molt. Plumage weight during the molt increased about 30%. Body feathers constituted the largest fraction (ca. 75%) of the plumage mass, even during molt. Plumage weight after molt was about 24% of the lean dry weight. Body lipid reached its nadir at the onset of molt in Fairbanks captives (LI = 3.5), but increased 6- to 7-fold at the end of molt (LI = 20). In Pullman outdoor captives, the increment was about 5-fold. Indoor captives vary in postmolt fattening, depending perhaps upon prior photoperiodic history before transfer to a constant photoperiodic environment. Thus some individual birds transferred later in the spring showed some degree of postmolt fattening, whereas those transferred earlier showed none. This is but a tentative conclusion. Although lipid reserves in the White-crowned Sparrow are small at the onset and perhaps most critical period of molt, the long feeding period and the mutual exclusion of the breeding and molting schedules preclude any apparent calorific drain on the avian body. It is suggested that the rapid molt period may be a selective force operating to separate temporally two events of the annual cycle that are potentially costly energetically.