During July and August woodlands and gardens are quiet, almost devoid of birdsong. The robin, and the blackbird that sang so boldly during spring are silent and skulking – they are moulting.
A bird’s feathers give it protection, colour and pattern for recognition, courtship and camouflage, and in most species enable it to fly. Birds must keep their plumage in good condition and spend much of their time bathing and preening. However, feathers, being lifeless structures, composed mainly of keratin, a mineralised protein, eventually wear out and must be changed. This is done by a process called moulting. The old feather is loosened by the growth of a new feather bud and falls out, allowing the new feather to replace it. Daylength provides an external stimulus to most birds, and hormones an internal stimulus.
Nearly all birds change all their feathers at least once a year. This is usually done after the breeding season and is called the post-nuptial moult and follows a set pattern in each species.
In most birds the complete moult starts with the inner primary feathers, matching pairs are dropped simultaneously so the bird can still fly, and progresses outwards so by the time the outer primaries are shed the new, inner ones will be fully grown. Part way through the primary moult, the outer secondary feathers are shed and the moult moves inwards. At the same time the inner pair of tail feathers is shed and the moult moves outwards. There are certain exceptions, treecreepers and woodpeckers keep the longer, stiffer pair of central tail feathers, has a means of balance when climbing trees, until all the other new tail feathers have grown. While moulting the flight feathers, the body plumage is also changed during the moult period.
In many birds there is a second moult before the breeding season and this pre-nuptial moult generally involves only the body feathers. Some birds assume breeding dress without a second moult. In autumn for example, male reed buntings and bramblings look rather sombre, but during the winter their feathers are abraded so that the dull tips and edges are worn away and appear as a fresh courtship plumage. Large birds, gulls for instance, often have a series of immature plumages until maturity is reached.
Water-birds, including ducks, geese and swans lose all their flight feathers within a short space of time and become temporarily flightless. Some other species have ‘moult migrations’, flying to rich feeding grounds where they can change their feathers in safety. Unique amongst perching birds, the dipper moults its primaries so quickly that it too may be flightless for a short period.
As feathers are so important for display and flight, the timing of moult must be linked to breeding and migration, and each species has adapted accordingly. For instance, birds of prey and swifts, which depend on flight to catch their food, have a prolonged moult so that their flying ability is not impaired. Long distant migrants usually moult before migration in order that their feathers are in good condition for their long journey. However, much energy is expended in feather renewal, and birds like swallows and some warblers delay moulting until they reach their winter quarters.
Yet another pattern is shown by the common tern, which starts to moult before migration, then arrests the process, to complete it after its migration journey. The willow warbler is unique among British birds in having two complete moults – one before and one after migration.
Differences in moult pattern even occur in the same species, one of our most elegant wading birds the ruff moults before migration, whereas its female counterpart, the reeve moults afterwards; the male blackcock has two moults a year, yet his mate only one. And who can say why the spotted fly-catcher, alone amongst all European species, moults primaries, secondaries and tail feathers in the opposite direction to all other species?