Moulting in the wrong season? A house sparrow replacing a significant part of its plumage in winter

Abstract

Extra moult episodes in wild passerines have rarely if ever been documented. Here, we report a case of a possible extra moult episode in a House Sparrow Passer domesticus, a species with just one moult per annual cycle. This bird was captured in active and intense plumage growth in Barcelona on 15 February 2021, c. five months after it had been ringed whilst undergoing its post-juvenile moult. This possible extra moult episode involved all body tracts except the crural, all tail tracts, and most wing tracts except the underwing coverts, primaries, primary coverts and alula. We found no evidence that this plumage replacement was the final phase of an interrupted post-juvenile moult. Hypotheses that could explain this extraordinary plumage growth include a mutation affecting the endogenous rhythm of the bird, severe stress, endocrine disruption or a predation event. Although the replacement of the wing feathers was highly asymmetric, we interpret this case as an extra moult episode of unknown cause rather than plumage regrowth after accidental loss (e.g. by predation), given that this plumage growth was compatible with three characteristics of moult not expected during the replacement of accidentally lost plumage: (i) it affected the whole of the bird’s plumage, (ii) involved feathers such as the axillaries that are rarely exposed, and (iii) showed sequential rather than simultaneous feather growth.