Like other finches Siskins can sometimes be aged on the shape of their tail feathers with juveniles being more pointed than the rounder feathers of the adult. This feature can't be used alone though as it has been known for young birds to accidentally lose their tails and grow and adult type replacement or, in some cases, moult their tails during their post-juvenile moult.
The photograph above shows a classic juvenile tail on the right compared to an adult one on the left.
The main focus on the studies in Barry's garden are the Blue Tits and Great Tits. By this time of year most young Blue Tits have moulted all their greater coverts but retain their greenish juvenile primary coverts. Normally these are moulted in sequence and we occasionally find a bird that has still retained one or more of the outer greater coverts which contrast with the fresh adult type. Very occasionally we find a bird like this one that has moulted its greater coverts out of sequence or accidentally lost one and grown a new adult type.