Seasonally divided moult in the Barred Warbler (Sylvia nisoria) is an endogenously controlled strategy

The seasonal timing of moult in migratory birds is an adaptation to cope with time constraints in the annual cycle. Kiat and Izhaki analysed moult patterns in Palaearctic passerines and rejected the proposition that seasonally divided moult is an endogenously controlled strategy. Instead, they advocated the view that it occurs due to a flexible and opportunistic timing of moult. In contrast, we argue that Kiat and Izhaki’s analysis is flawed and that they overlooked several important facts about moult in the Barred Warbler Sylvia nisoria and other species showing seasonally divided moult. These include the facts that juveniles replace a few secondaries already in their first winter, and that the moult sequences of primaries and secondaries are decoupled compared with the typical passerine complete moult sequence. We argue that seasonally divided moult is an adaptive strategy that is largely under endogenous control.