Carbon, Nitrogen, Ash, and Caloric Density of the Lean Dry Body Mass of White-Crowned Sparrows during Postnuptial Molt

Abstract

Studies of postnuptial molt in the migratory Whitecrowned Sparrow (Zonotrichia leucophrys gambelii) have revealed complex physiological and behavioral patterns that superficially include a reduction of lipid reserves (King et al. 1965), an increase in blood volume (Chilgren and deGraw 1977), and increases in total body water without attendant changes in the lean dry mass (LDM; Chilgren 1977; see also King 1980 for a summary of metabolic adjustments associated with molt). Preservation of the LDM in molting birds presupposes a level of nutritional adequacy or lipid stores to support plumage growth while maintaining normal body function. A behavioral component, depressed perch activity levels, is also observable in captive and free-living birds. While the function of this activity change is not clear, reduced activity may serve to minimize caloric expenditure or attract fewer predators to a passerine ill-equipped for rapid flight (Chilgren 1975). Previous analysis of the body composition of this species during postnuptial molt has included lipid reserves, dry plumage weight, plumage water, body water, and the lean dry mass (Chilgren 1977). I subsequently analyzed the LDM for carbon, nitrogen, ash, and caloric density. Any structural or chemical modifications occurring within the relatively weightstable LDM during molt in captives might then be identified by these techniques. Two populations of birds were studied: those mistnetted in central and eastern Washington during the fall or spring migrations and a small group (n = 13) of birds captured in late June at Fairbanks, Alaska (64?50'N) after the breeding season but before the molt. All Washington birds were eventually transferred to outdoor aviaries at Pullman, Washington (46?46'N) prior to the division of these birds into three groups of individually caged birds. Two groups were placed in constant conditions of light (LD 16:8) and temperature (5?C or 15?C), while the third was outdoors and shaded. All birds were provided with a chick-starter mash and water ad libitum. The small group of Fairbanks birds, which are difficult to obtain in large numbers at the breeding site before the molt, was inadequate for body composition study during molt and contributed to data either before (stage 1) or after (stage 5) molt. Hence, all data from birds during molt were derived from the three Pullman groups. Studies of the tempo and duration of post-