Split moult: a response to time constraints during the main moult period

Abstract

Split moult is a relatively rare adaptation to cope with time constraints in the annual cycle, although less rare than previously thought. This moult pattern mainly occurs among long‐distance migrants but also among some northern species wintering in the northern or temperate zones. Hedenström et al. rejected the conclusion of Kiat and Izhaki that split moult is a time constraint strategy; instead, they advocated the view that it is an endogenously controlled strategy. They based their conclusion on the first winter secondaries moult by juvenile birds and on the post‐breeding moult sequence. In contrast, we argue that Hedenström et al.'s conclusions are flawed, probably due to their focus on only one species (Barred Warbler Curruca nisoria). Our results suggest that there are no indications of the uniqueness of the secondaries moult by juvenile birds in relation to other species which have no split moult. We thus suggest that split moult is not initiated during the birds' first winter. In addition, there is no difference in moult sequence, although decoupling between two feather tracts occurs (primaries and secondaries), which is a common mechanism among many passerines that perform a partial or incomplete moult. We therefore argue that split moult is an adaptive strategy driven largely by the actual time constraints the birds experience, as already experimentally proven.