A detailed analysis of primary feather moult in the Common Starling Sturnus vulgaris– new feather mass increases at a constant rate

During moult, the rate that protein can be synthesized and used to grow new feathers must be a critical factor influencing the quality of new plumage. However, the rate that new feather mass accrues during moult has not been assessed in detail. To estimate this, the increase in length of each of the nine primary feathers (P1–P9) in 16 captive Common Starlings Sturnus vulgaris was measured at weekly intervals throughout moult. The rate of increase in length was similar for all primary feathers except P9, which increased more slowly. After completion of moult, these feathers were plucked, accurately measured and weighed. The distribution of mass along the length of each primary was assessed. Using these data, I estimated the rate that mass had increased for each feather during moult. For each individual primary, mass increased at a steady rate until almost fully grown. The rate of increase in mass was least for P1 and greatest for P9. The number of feathers growing concurrently decreased as moult progressed. The net result was that total new primary feather mass increased at a steady rate throughout most of moult (linear regression between start of growth of P2 and end of growth of P8; r2 = 0.991). Retrospective conversion of feather lengths into moult score showed that the increase in score was not linear. It was greatest early in moult and decreased as moult progressed. A scoring system that factors in distribution of mass within and between feathers may provide a more physiologically relevant measure of the progress of moult.