Wing moult in relation to autumn migration in adult Common Whitethroats Sylvia communis communis

Most long-distance passerine migrants in Sweden moult on breeding grounds before leaving on autumn migration to winter quarters. However, birds laying second or replacement clutches, or just breeding late, have too little time for a normal moult on the breeding grounds. When time is limited the birds may respond by making various adjustments to the moult, for example by moulting more quickly or by suspending the moult. In this study, the relationship between the performance of post-nuptial remex moult in Common Whitethroats breeding on Gotland, southeast Sweden, and autumn migration departure was investigated. The majority (77%) of the birds had interrupted moult in either the primaries or secondaries. Interruption of moult was more common among birds with a later onset date, as was asymmetry in moult between wings. The interruption of moult led to a significant time gain and moult completion was, consequently, more synchronized than moult onset. The results from this study indicate, in accordance with other data, that an early start of autumn migration is important. An early start may be crucial to facilitate the crossing of the Sahara Desert once the dry season has begun.