Sulfur Amino Acid Nutrition during Molt in the White-Crowned Sparrow. 2. Nitrogen and Sulfur Balance in Birds Fed Graded Levels of the Sulfur-Containing Amino Acids

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Abstract
In an effort to estimate a critical concentration of dietary S-containing amino acids (SAA) for molting birds, N and S balances were measured before, during and after postnuptial molt, in 5 groups of white-crowned sparrows (Z. l. gambelii) fed isocaloric and isonitrogenous (17.4 kJ/g and 13.5% protein) diets, in which SAA concentration ranged from 0.33-0.78%. These SAA concentrations approximate those found in natural diets and bracket the SAA requirements of poultry. Birds from all dietary groups showed positive N and S balances through 6 temporally equal stages of molt, and in the nonmolting periods. N retention differed significantly among molt stages, but not among dietary groups. The grand average net N retention during molt (total N retention minus basal N retention) was 623mg/25g bird. S retention differed significantly among stages and among dietary groups. Because group differences occurred during molt and in nonmolt, the net S retention during molt (total S retention minus basal S retention) did not differ significantly among dietary groups. The grand average net S retention during molt was 85.4 mg/25g bird. These results indicate that molting white-crowned sparrows are typically able to meet the protein and SAA demands of molt through their food (the critical dietary SAA concentrations being less than those likely to be found in natural diets), thus precluding the need to deplete tissue proteins.