The garden has been teaming with Goldfinches over the last few months, with the good numbers of young suggesting it has been a successful breeding season for these delightful little birds. Initially, the young Goldfinches could be told from their parents by their lack of the characteristic red and black facial markings. By now, however, they have moulted through into their adult plumage and can no longer be distinguished by eye, as I watch them on the feeders through the kitchen window.
In the hand, when caught during a ringing session, it is possible to identify juvenile Goldfinches by looking at subtle differences in the shape of the tail feathers and the colour of certain wing feathers. Such differences come about because of the way in which these birds renew their feathers during moult. In most small birds that breed and winter here, the adults undergo a complete renewal of their feathers once the breeding season is over. Some species undergo a second, ‘partial’, moult just before the start of the next breeding season. Many of our summer-visiting migrants, however, will not undergo their annual moult until they have reached their wintering grounds, although some start the moult here before interrupting it just ahead of their long journey south.
Most young birds follow a different strategy, replacing some feathers after the breeding season and growing others for the first time, but not acquiring full adult plumage until after the next breeding season is over. You can see this for yourself by looking carefully at male Blackbirds in the early summer. If you spot a male Blackbird who is all black apart from a few dark brown feathers in his wing, then he will be a bird born the previous year.
Getting young birds out of the nest quickly is an important strategy because it reduces the period when they may be particularly susceptible to predation. Young birds grow rapidly in the nest and need to be able to fly when they fledge. Because of this, the young tend to invest relatively little in the growth of body feathers, concentrating their efforts instead on the all-important flight feathers. As a result, newly-fledged young often look rather fluffy and ‘loose feathered’, but they will soon grow the remaining feathers, often acquiring adult body plumage over a period of several weeks. These different moult strategies are useful for me, as a ringer, since they enable me to age most birds with relative ease. There are, however, some species where the young moult through into full adult plumage soon after leaving the nest – House Sparrow and Long-tailed Tit are two examples – and these birds cannot be aged once they have gone through the autumn moult.