Passerines moult during various life-cycle stages. Some of these moults involve the retention of a variable quantity of wing and tail feathers. This prompts the question whether these partial moults are just arrested complete moults or follow different processes. To address it, I investigated whether three relevant features remain constant across partial and complete moults: 1) moult sequence (order of activation) within feather tracts (e.g., consecutive outward moult of primaries) and among tracts (e.g., starting with marginal coverts, followed by greater coverts second, tertials, etc.); 2) dynamics of moult intensity (amount of feathers growing along the moult progress); and 3) protection of wing quills by overlapping fully grown feathers. To study the effect of moult completeness on these three features, I classified moults of 435 individuals from 61 species in 3 groups: i) complete and partial, ii) without and iii) with retention of feathers within tracts. To study the effect of life-cycle stage, I used postbreeding, postjuvenile, and prebreeding moults. I calculated phylogenetically corrected means to establish feather-moult sequence within tracts. I applied linear regression to analyse moult sequence among tracts, and polynomial regression to study the dynamics of moult intensity as moult progresses. Sequence and intensity dynamics of partial moults tended resemble those of the complete moult as moult completeness increased. Sequence within and among feather tracts tended to shift as moult intensity within tracts and number of tracts increased. Activation of primaries advanced in relation to the other feather tracts as number of moulted primaries increased. Tertial quills were protected by the innermost greater covert regardless of moult completeness. These findings suggest that moult is a self-organised process that adjusts to the degree of completeness of plumage renewal. However, protection of quills and differences among species and between postjuvenile- and prebreeding-moult sequences also suggest an active control linked to feather function, including protection and signalling.

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.