The good oil that is now being posted about moult should make it clear that
changes in wing shape through moult to facilitate migration - the
proposition originally posed - just do not happen. Many species of bird
can, however, change the shape of their wings with substantial changes to
their aerodynamical properties simply by the extent to which they use them
when flying.
The outer primaries of many species have emarginations or notches.
Emarginations are a gradual narrowing of the outer (leading) web of the
feathers towards the tip of the feathers, often starting a third to a half
of the way from the tip to the base of the feather. Notches are a sudden
narrowing, again up a half the feather length, on the wider trailing web of
the feathers. The number of feathers with emarginations and/or notches
varies between species.
Birds can certainly twist their primaries and they can decide how much to
open their wings. Either of these actions would allow the bird to expose
the notches and emarginations thus allowing air to slip through the gaps;
this would slow the bird down and give increased manoeuvrability. If the
wings are not twisted and not fully opened, the gaps will not be exposed
and the birds will fly faster and straighter. The former option would be
an obvious advantage when feeding, say, for a flycatcher, and the latter on
migratory flights.
I know of no literature on the subject; is there any?
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