The Grasshopper Warbler (Locustella naevia) is an example of a species in which the adults complete a partial post-breeding moult (unlike Reed Warblers that will replace body feathers but not flight feathers - with a few exceptions). The birds complete the moult on the breeding grounds once they have finished breeding which is often the determining factor in the extent of the moult. Food availability and environmental conditions also influence the amount of energy that a bird can dedicate to moulting.
This Grasshopper Warbler has moulted its tertials, the innermost primary and all but one of its greater coverts.
Grasshopper Warbler (Locustella naevia)
As all of the Grasshopper Warblers that we catch here in Portugal are migratory, the birds are often carrying large fat deposits that they need to provide energy in order to make the long crossing across the Sahara. Last week we captured a bird with a mass of 19.7g which is impressive given that the lean mass of a Grasshopper Warbler is between 11 and 13g.