The energy cost of feather replacement is not intrinsically inefficient

Feathers serve a diversity of functions in birds and their continuous use and exposure to the environment requires a scheduled moult to maintain their full functionality. As feathers represent about 25% of a bird’s protein content, moult is expected to impose substantial energy and nutrient demands, but perhaps not to the extent reported. Energy conversion efficiencies for feather formation are among the lowest for any biological structure examined, but this assumes that increases in maintenance energy requirements (minimum resting metabolic rate (RMRmin)) during moult are predominately due to feather synthetic costs. We tested this assumption by comparing the RMRmin and protein turnover rates of House Sparrows (Passer domesticus (Linnaeus, 1758)) during peak moult and in a non-moulting cohort before and 12 days after having a similar amount of feathers plucked. Replacement of plucked feathers had no effect on metabolic rate, whereas RMRmin was 28% higher in moulting than in non-moulting House Sparrows. Protein turnover rates were lowest in non-moulting birds, but rate differences between non-moulting and moulting birds were threefold higher than those between non-moulting and plucked House Sparrows. Thus, the energy inefficiencies reported for feather replacement are mainly due to costs associated with coincident processes rather than being a direct cost of feather synthesis per se.