Just as we replace our clothes when they become old and tatty, so birds do the same thing with their feathers.
These are essential for insulation, flight and manoeuvrability, and for helping to evade their natural predators.
A bird's feather is an amazing and intricate structure comprising thousands of parts called barbs and barbules that hook up and lock together. It is strong yet flexible and comes in different lengths, shapes and colours to fulfil the needs of each species.
Downy-edged feathers provide warmth and insulation while the long, more rigid primary flight feathers give lift and forward thrust. A small bird may have two or three thousand feathers, whereas a swan can have up to 25,000. The greatest concentration of feathers is in the head and neck. Bathing and preening will help keep feathers in good condition but they only have a blood supply while growing. The mature feathers are actually dead structures and they will eventually wear out and have to be replaced.
The length of this moult and its timing will vary depending on the species. For instance, far-travelling migrants and birds that frequently fly through trees and bushes suffer greater wear and tear and tend to moult twice a year. Those living in more open spaces usually moult once a year.
Autumn is the main moulting season as breeding has finished and offspring are fending for themselves. The days are still warm and food is usually plentiful. As they have to retain the power of flight, small birds will moult only a few feathers at a time, with the whole process taking from five to 12 weeks to complete. A large raptor may take up to two years to replace all its feathers.
Female peregrines and ospreys will have a partial moult while incubating and many migrants will undertake a partial moult at the end of the breeding season and complete their moult on their wintering grounds.
Some birds are "synchronous moulters" - they shed their flight feathers simultaneously - and certain birds on the Basin do this. The eider, merganser and great crested grebe will be flightless for up to four weeks and the mute swan for up to seven weeks.
The vastness of the Basin is a safe haven at this vulnerable time and also provides a vital larder for the birds to feed in. Moulting is a stressful and energy-consuming process so the availability of a good food source is essential to sustain the development of new feathers. During this time birds conserve energy by being less active and will try to be as inconspicuous as possible.
Robins are a good example of this. You will seldom see any during June and July but by the end of August they start popping upon prominent perches, singing away and flaunting their new plumage.