We examined the extent of postjuvenile moult of rectrices, greater coverts, alula and tertials in the Great Tit Parus major during migration and feather replacement near Lake Ladoga. The moult of 16 hand%raised birds was also followed in experimental photoperiodic conditions simulating different hatching dates. The data from caged birds showed that the extent of moult in the Great Tit is photoperiodically controlled. Autumn short%light days reduce the extent of moult. The data from moulting birds trapped in the wild agreed with experimental results and revealed that the timing of feather replacement influenced the individual extent of moult. Annual variation in the extent of moult was influenced by the timing of breeding in the population. A possible explanation for the decline in the extent of moult during migration is discussed.