Feathers are incredibly tough. However, despite this, and frequent washing and preening, they will eventually wear out, becoming discoloured or damaged. Feathers do not grow in the same way as human hair or nails do, and they must be replaced naturally through the moulting process. As older feathers become loose, fresh new feathers will push through the follicles and dislodge them.
In a lot of species, especially the smaller birds, moulting first occurs in the autumn after they hatch, and it is at this time that the juvenile feathers are replaced with the first winter plumage. This is usually not a full moult, but the outer wing, head and body feathers will be shed and replaced.
There are species for which the first winter feathers are no different from the adult feathers. Conversely, for a variety of species there may be quite a few differences between the eventual plumage of the adult bird and the first winter plumage. Blackbirds, for example, will retain some brown wing and tail feathers as juveniles, before loosing them in adulthood, to be replaced with their black plumage. Other species can take significantly longer to make the transition from immature to adult plumage. Gulls and other sea birds may not develop their adult feathers for around four years.
It is also not unusual for some species, such as the greenfinch and the lapwing, to have different feathers for different seasons. During the breeding seasons, the adult male bird will often have brighter, showier feathers in order to attract the female. Each time their feathers change, the bird will almost always go through the process of moulting.
Moulting costs the bird a significant amount of energy, so it usually takes place in the late summer months when there is still enough wild bird food and the weather is still quite mild. Even so, the moulting usually occurs over a period of time, rather than suddenly all at once. For smaller species, such as the blue tit, a full moult might take about six weeks, while larger birds will take a lot longer. The herring gull is known to moult over six months and the buzzard over a number of years. Most species will lose the flight feathers in the tail and wings in a very specific order. However, this order differs from species to species.
Supplying wild bird food can be really helpful for garden birds and helps them during times of added stress or when they require extra energy.