New feathers, please
15 August 2006
In late summer it can feel like the birds have simply disappeared.
There's no song - the garden and countryside can be eerily quiet - and few birds to see. What's more, when birds do appear they can look quite strange - yellowish blue tits, blackbirds with speckled heads, brown-headed starlings.
But there is a simple explanation for these observations. Birds are moulting.
Moulting is a process of shedding and regrowing feathers. Adult birds are shedding their worn out feathers from this year's breeding season and growing new, strong, warm feathers to see them through the winter. This year's young are losing their first feathers and moulting into their adult coats.
'A full moult may be spread out over a considerable time period, which is fine if there is a plentiful food supply, and if the bird is not a migrant'
Feathers wear out during a bird's busy year. Flying, rubbing against neighbouring feathers or trees, general weakening due to exposure to sun, along with parasites, such as feather lice, all cause damage to feathers.
A comparison of feather wear shows that pigmented (dark) feathers wear more slowly than white ones.
Feathers grow from follicles in the skin (like hair) and the growth of a new feather from the bottom of the follicle pushes the old one out. The process is a gradual one and occurs in sequence across an area of skin to ensure that there are no ‘bald’ patches.
This means that a full moult may be spread out over a considerable time period, which is fine if there is a plentiful food supply, and if the bird is not a migrant.
Patterns of moult depend upon species, age and time of year but most birds will moult completely during a year, sometimes split into two or three moult periods, usually before and after breeding. Small birds take about five weeks to moult and regrow their flight feathers, with migratory species being the quickest.
How does moulting affect birds?
Moulting is a drain on a bird’s resources. It takes energy to grow new feathers, there may be heat loss when feathers are shed, affecting insulation, and when flight feathers are lost, more energy may be needed for flight.
This is the reason why many birds become inconspicuous for a time as they may be more vulnerable to predation during their moulting period. Generally, moult will not overlap with other processes which are a drain in a bird’s energy, such as breeding or migration.
Unlike most other birds, ducks, geese and swans lose all their flight feathers at once, rendering them flightless for a period.
In ducks, to provide some protection for the brightly-coloured males, the moult starts with their bright body feathers. These are replaced by dowdy brown ones, making them look much like females.
This eclipse plumage is why in mid- to late summer, it seems that all the drakes have gone. Once the flight feathers have regrown, the birds moult again, and by October the full colours have been regained and the various species of ducks are easily recognisable.
Female ducks lose their flight feathers later, after the young become independent. Shelducks have a ‘moult migration’, flying to a safe area to undergo their moult before returning.
Birds which breed in the Arctic, with its short summer, will often start moulting before the young have fledged and birds like the snow bunting may become flightless, enabling all the flight feathers to be renewed at once, in a short period of time.
Some migrants will moult before they migrate, while other wait until afterwards.
Moulting in young birds
Some young birds undergo what is called a partial moult - young robins are a good example. The ‘spotty’ juvenile body feathers are moulted in the late summer, being replaced with the typical orange breast and brown back of an adult, while the wing feathers are retained and not moulted for a whole year.
The gradual replacement of feathers in young birds can lead to some strange looking individuals: blackbirds which look sleek and dark, except for a spotty brown head (the last feathers to moult), or starlings which have gained a white-spotted winter plumage on their bodies but still have the pale brown head of a youngster.
Some birds moult more than just feathers - puffins actually shed the colourful horny plates which cover their bills and regrow them ready for the next breeding season.