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 small compared with the energy expended for molt, resulting in a low energetic efficiency for keratin deposition (as low as 3%). We measured the fractional synthesis rates (FSR) of tissue and whole-body protein in molting and non-molting sparrows using the ''flooding dose'' method to determine if part of the energetic cost of avian molt might be accounted for by an accelerated rate of whole-body protein turnover. Molting sparrows had significantly higher FSR of protein in liver, muscle, and whole-body (excluding the integument) throughout the 24 hr cycle compared with values for non-molting sparrows. In absolute terms, molting sparrows synthesized daily 260 mg of body protein (excluding keratins) 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 day (75 mg) during the most intensive phase of molt. This accelerated rate of whole-body protein synthesis undoubtedly contributes significantly to the energetic costs of molt.