Moult intensity constraints along the complete moult sequence of the House Sparrow (Passer domesticus)

Sequence and intensity are two essential components of bird moult. While the moult sequences of remex tracts are highly homogenous across passerines, other tracts apparently show a high variability. Moreover, order of moult activation among tracts are insufficiently known. Likewise, dynamics of moult intensity as moult progresses remains poorly known. Here, we provide detailed quantitative description of moult sequence and intensity in the House Sparrow (Passer domesticus). To understand their role, we tested two hypotheses on the: 1) protection function of moult sequence, and 2) aerodynamic and physiological constraints on moult intensity. We scored percentage growth of 313 captured sparrows using the mass of the feathers of each tract (also length for remiges) to monitor moult intensity throughout the complete moult progress, which is defined as the fraction of new and growing feathers in a moulting bird relative to the total plumage. Moult sequence was highly variable both within wing coverts and among feather tracts, with moult sequence differing among all birds to some degree. We only found support for the protection function between greater coverts and both tertials and secondaries. Remex-moult intensity conformed to theoretical predictions, therefore lending support to the aerodynamic-constraint hypothesis. Furthermore, remex-moult speed plateaued during the central stages of moult progress. However, overall plumage-moult speed did not fit predictions of the physiological-constraint hypothesis, showing that the remex moult is only constrained by aerodynamics. Our results indicate that aerodynamic loss is not simply the inevitable effect of moult, but that moult is finely regulated to reduce aerodynamic loss. We propose that the moult of the House Sparrow is controlled through sequence and intensity adjustments in order to: 1) avoid body and wing growth peaks; 2) fulfil the protection function between some key feather tracts; 3) reduce detrimental effects on flight ability; 4) keep remex sequence fixed; and 5) relax remex replacement to last the whole moult duration.