AMINO-ACID COMPOSITIONS OF AVIAN EGGS AND TISSUES - NUTRITIONAL IMPLICATIONS

To establish a framework for detailed investigations of the roles of protein nutrition and metabolism as proximate factors limiting avian egg production, I analyzed the amino acid compositions of the mixed proteins in the albumen and yolk of eggs of Zebra Finches Taeniopygia guttata and Pigeons Columba livia and compared these data with analyses of eggs from three other avian Orders. I also analyzed the amino acid compositions of muscles and carcasses of a representative passerine, the White-crowned Sparrow Zonotrichia leucophrys gambelii and compared these analyses with analyses available for other species. The amino acid compositions of the mixed proteins of albumen and yolk were very similar, as were the amino acid compositions of eggs or tissues among the species examined. Using the aforementioned measures, I compared the apparent essential amino acid (EAA) requirements for egg production (i.e. the EAA deposited) with (1) the estimated daily EAA requirements for maintenance in nonproductive birds, (2) the amino acid composition of the mixed proteins of muscle (a source of amino acids during fasting or periods of trophic shortages) and (3) the average amino acid concentrations in several food types. For all species examined, during egg production, the daily requirements for Phe + Tyr increased most among the EAA, followed by Lys or Val. The sulfur amino acids (SAA) and Phe + Tyr, or Trp were the first and second limiting amino acids, respectively, for synthesizing egg proteins from amino acids derived from muscle proteins. The SAA and Lys were the most likely to be limiting for synthesizing egg proteins from dietary proteins. Even the most prolific passerines can probably meet their SAA demands for egg production without highly selective foraging, but some selective foraging may be needed. Many ducks, however, may need to forage intensively for SAA-rich foods to avoid severely depleting their body protein reserves and (or) very inefficient use of EAA and energy.