The rapid moult strategy of nominate fuscus makes it difficult to distinguish all sub-adult birds from adults. In some individuals it may be difficult to distinguish 3cy birds from 4cy. Lars Jonsson's article in Birding World shows the plumage characteristics in detail. In 3cy spring birds, fuscus normally shows a pale yellow bill, already a red gonydeal angle and the iris is pale yellow. Primary moult may start again in 3cy birds in March and result in a clear division of old third generation and new fourth generation inner primaries when fuscus returns at northern latitudes in summer. Most often, a clue of immaturity can be found in the tail; a few tail feathers have been moulted and are clear white, some older feathers show black spaghetti or vermiculated pattern. In 4cy birds, except from some odd black markings in the tail, these sub-adults are hard to tell from adults.