The Norsey Wood Society works closely with Basildon Borough Council, the owners of Norsey Wood Nature Reserve, an ancient woodland which is also
a Site of Special Scientific Interest on the edge of Billericay in Essex, UK
Songbirds-Moult
Songbirds - Exploring the Mysteries of Moult
The following is an overview of a very complex subject. A subject that fosters scientific papers bedecked with bar and pie charts but are avoided here! It is also a subject that is fraught with ‘except for the exceptions’! Aspects of the natural world often refuse to be neatly pigeonholed!
Feathers are dead material made out of protein keratin just like our finger nails. They are an expression of what the bird was eating at the time of replacement and if the bird struggles to feed, fault bars occur across the vane as the feather does not stop growing. Feather maintenance and replacement is essential for the survival of birds. Moult is triggered by the seasonal changes and has to be slotted within the bird’s annual cycle. Apart from the wing and tail feathers, the contour and body feathers are also replaced annually. Some birds undergo a further moult whereby they moult their body feathers to bring them into courtship plumage ready for spring (Buntings, finches and warblers in particular). This is also achieved by the tips of the body feathers being abraded to reveal the brighter colours beneath (Bramblings, Siskins and Redpolls are a good example of this).
Basically, with reference to adult birds, moult starts in the centre of the wing and progresses outwards (the primary feathers) soon to be followed by the secondary feathers that progress inwards towards the body. There are 10 primaries and 9 secondaries that form the flight feathers of a typical songbird. This is achieved in such a way that replacement feathers are re-growing in the gaps and thus minimize the loss of flight efficiency. The tail moult commences with the loss of the two central tail feathers and progressively moves outwards. The simultaneous wing and tail moult can take up to three or more weeks to complete and involves a considerable expenditure of energy. In contrast, most moulting wildfowl become flightless for several weeks and unless they are in a safe habitat they become vulnerable to predation.
Wing moult in a Goldfinch:-
From left to right
- Outer 4 primaries await replacement (The outer 1st primary is too small to see)
- 1 new primary growing in
- 5 new primaries fully replaced, fresh and glossy
- 2 new secondaries growing in
- 7 secondaries await replacement.
[Photograph by Greg Conway – BTO]New paragraph
Resident birds usually go into wing moult after their breeding duties whilst most migratory songbirds (warblers in particular) delay their moult until they reach their winter quarters in Africa. As songbirds’ flight becomes temporarily inefficient they tend to skulk about the undergrowth for several weeks until their wing and tail moult are complete. During this time usually only contact calls, alarm calls and the muted subsongs are heard as feather replacement saps the bird’s energy. A time to assume a low profile and avoid the attention of predators!
Sometimes in the autumn, crows flying overhead can be seen to be in moult whereby there are significant gaps in the wings and tail. Thus the bird has to work harder to stay aloft. Birds with excessive gaps in the wings and tail are described as being in ‘far flung moult’!
Although we label some garden bird species as ‘resident’ (Blackbirds, Robins, House Sparrows and Goldfinches for example) after moulting many will undertake local movements or they may even travel further afield. For example, some our Robins may seek a better winter climate in France or Spain whereas others will keep to their garden territory. And some Blackbirds and Robins from continental Europe will brave the dangers of a North Sea crossing to seek a better winter climate in the UK. Our garden Blackbird and Robin, are not always the same one! That is to say, that within a species, some are partial migrants.
Some migratory songbirds, Common and Lesser Whitethroats in particular, whose foraging and breeding lifestyle involves them skulking through dense abrasive scrub undergo so much feather wear that wing and tail moult are essential before attempting their return flight to south of the Sahara.
Two curious moult strategies that are yet to be explained are that of the Spotted Flycatcher and the Willow Warbler. The former moults its wing feathers in the opposite direction (reversed moult) to that described above and the latter has two wing moults – one post breeding and another whilst wintering in Africa. The energy requirements to achieve two moults must justify its need for flight efficiency.
As the Arctic ice sheet retreats (a natural cycle but accelerated by Man) it opens up further northerly breeding locations thus long distant migratory species are having to fly a longer distance between their African winter quarters and their European and Arctic breeding sites. Presumably feather maintenance, especially in terms of moult, will need to keep abreast of the extra demands required.
Once one looks into the moult strategy of the other bird families, gulls and seabirds, wildfowl, birds of prey, waders and owls, a whole new world of moult strategy complexity is opened up and more exceptions to the rule emerge!