Unscheduled plumage replacement in passerines: identification and implications for molt regulation

Molt is the periodic replacement of a bird’s plumage within the annual cycle. Replacements outside the known annual molt strategy of each species are considered accidental. During the winter of 2023–2024 in Barcelona city, we found evidence of a second plumage replacement in birds of three passerine species with only one known molt in their annual cycle: House Sparrow and Blue and Great tits. To evaluate how this fits our understanding of molt, we first considered whether these feathers could result from a delayed molt or from a new replacement occurring after the species’ scheduled molt. We ruled out the possibility of a delayed scheduled molt because the plumage was in fresh condition. We therefore examined whether these replacements could be treated as molt or as accidental feather replacement. To do so, we quantified two components of the molt process (intensity and sequence) and two components of the molt output (extent and asymmetry). Values of the four molt components were lowest in the House Sparrow, intermediate for the Blue Tit, and highest in the Great Tit. Using values of molt components, we propose three rules to determine whether unscheduled plumage replacements should be regarded as an actual extra molt episode or just accidental plumage replacement. From these rules, we concluded that the House Sparrow exhibited an accidental replacement or a highly asymmetrical extra molt episode, whereas both tit species were undergoing an extra molt episode, though asymmetrical in the Blue Tit. Unscheduled plumage replacement may not be an adaptive trait in some species, but could facilitate the evolution of more complex molt strategies and new plumage functions in other species.