SPARROWS INCREASE THEIR RATES OF TISSUE AND WHOLE-BODY PROTEIN-SYNTHESIS DURING THE ANNUAL MOLT

Avian molt is a period of adult growth involving mainly the deposition of keratins. The energy contained in newly synthesized plumage is sma ll compared with the energy expended for molt, resulting in a low ener getic efficiency for keratin deposition (as low as 3%). We measured th e fractional synthesis rates (FSR) of tissue and whole-body protein in molting and non-molting sparrows using the ''flooding dose'' method t o determine if part of the energetic cost of avian molt might be accou nted for by an accelerated rate of whole-body protein turnover. Moltin g sparrows had significantly higher FSR of protein in liver, muscle, a nd whole-body (excluding the integument) throughout the 24 hr cycle co mpared with values for non-molting sparrows. In absolute terms, moltin g sparrows synthesized daily 260 mg of body protein (excluding keratin s) above that synthesized by non-molting sparrows. The daily increase in whole-body protein synthesis in molting birds equaled at least 3.5- fold the amount of protein synthesized and deposited as keratin per da y (75 mg) during the most intensive phase of molt. This accelerated ra te of whole-body protein synthesis undoubtedly contributes significant ly to the energetic costs of molt.