Abstract:
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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. |