From partial to complete: Wing- and tail-feather moult sequence and intensity depend on species, life-cycle stage, and moult completeness in passerines

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.