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

Abstract

-In an effort to estimate a critical concentration of dietary sulfurcontaining amino acids (SAA) for molting birds, we measured nitrogen (N) and sulfur (S) balances before, during, and after postnuptial molt in five groups of White-crowned Sparrows (Zonotrichia leucophrys gambelii) fed isocaloric and isonitrogenous (17.4 kJ/g and 13.5% protein) diets in which SAA concentration ranged from 0.33 to 0.78%. These SAA concentrations approximate those found in natural diets and, as well, bracket the SAA requirements of poultry. Birds from all dietary groups showed positive N and S balances through six temporally equal stages of molt and in the nonmolting periods. Nitrogen 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 623 mg/ 25-g bird. Sulfur retention differed significantly among stages and among dietary groups. However, because group differences occurred both 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/25-g bird. These results indicate that molting Whitecrowned 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. Avian molt is a type of growth characterized primarily by protein accretion in the form of feathers, and secondarily by the regeneration of other epidermal structures (Spearman 1966, Payne 1972, Chilgren 1975). Despite its morphological restriction, this integumental growth can entail a deposition of a protein mass approaching, or even exceeding, one-fourth of the protein content of a bird's body (Mitchell et al. 1931, Newton 1968, Myrcha and Pinowski 1970, Gavrilov and Dolnik 1974, Chilgren 1977, Carey et al. 1978). Moreover, the cyst(e)ine content of the plumage is disproportionately large compared to its content in the mixed proteins of other tissues or foods (Nitsan et al. 1981, Murphy and King 1982a; cf. F.A.O. 1970, Scott et al. 1982). These observations have, for decades, led avian biologists to question the ability of birds to fulfill the protein demands, or more specifically the sulfur amino acid (SAA) demands, of molt through daily food intake alone. The availability of dietary SAA has often been invoked as a potential explanation for observed changes in body composition, feeding habits, or even energetics during molt (e.g., Hanson 1962, Newton 1968, Gavrilov and Dolnik 1974, Brake et al. 1979), on the assumptions that (1) endogenous SAA reserves will be used if exogenous sources are inadequate, (2) birds may choose foods on the basis of SAA content, or (3) food intake may be increased above caloric needs to fulfill SAA requirements. The results of the few empirical investigations that have been conducted on these subjects thus far are equivocal. Ackerson and Blish (1926) reported a disproportionate nitrogensparing effect of supplemental cystine in molting domestic fowl (Gallus gallus var. domesticus) that were fed otherwise N-free diets. Subsequent experiments (Ackerson et al. 1928) using a corn/casein diet and cystine supplements, however, failed to show a beneficial effect of these supplements on body weight, egg production, or the dynamics of molt in fowl. Similar investigations involving a laying ration and methionine supplementation also were ineffective in shortening the molting period or in increasing post-molt egg production in fowl (Taylor and Russell 1943). Likewise,