Sulfur Amino Acid Nutrition during Molt in the White-Crowned Sparrow. 1. Does Dietary Sulfur Amino Acid Concentration Affect the Energetics of Molt as Assayed by Metabolized Energy?

Abstract

To test the hypothesis that the dietary intake of the sulfur-con- taining amino acids (SAA) methionine and cyst(e)ine affects energy intake or the allocation of energy to production and oxidation, respectively, we fed five groups of White-crowned Sparrows (Zonotrichia leucophrys gambelii) five isocaloric, isonitrogenous diets before, during, and after their postnuptial molt. The diets ranged in SAA content from sub- to superadequate in relation to standards es- tablished for poultry, and from very low to very high in relation to the SAA content of natural foods. The patterns of plumage replacement were indistin- guishable among the five groups. The body weights of all birds decreased sharply at the onset of molt, as is typical for this and other species, and then gradually increased through early molt and stabilized during mid- and late molt. The com- ponents of energy balance (gross energy intake, energy utilization coefficient, ca- loric density of excreta, total excretory energy output) did not differ significantly or consistently among the five groups. Likewise, the net energy of molt, as esti- mated by subtracting the maintenance requirement for metabolized energy from the expenditure during molt by the same birds, ranged from 582 to 611 kJ, but without significant differences among the groups and without consistent corre- lations to the diets. The grand average was 605 kJ/25-g bird. Hence, we are unable to verify the predictions of the amino acid model for the determination of the energy requirements of molt.