I am not aware of any birds that change their wing-shape or size through
moult between migratory and non-migratory phases of their annual cycle.
This I would categorise as seasonal variation of wing-shape. The component
of wing-shape that could be altered by moult is determined by the wing's
flight feathers (the remiges), i.e the relative lengths of the primaries
and secondaries. Therefore, for a species to have seasonal variation in
wing-shape it would need to moult its remiges (or some of them) at least
twice a year. Very few species do this. The three most common moult
strategies (which seem to be employed by the vast majority of species) are:
(1) one complete moult per year (all feathers replaced once); (2) one
complete moult and one partial moult per year (flight feathers repalced
once in the complete moult and contour feathers [head, body and
wing-coverts] replaced twice); and (3) "staffelmauser" or slow continuous
moult (feathers are more or less continuously repalced at a slow pace, once
a year or less, and distinct "generations" of feathers do not occur. In
none of these cases are the remiges repalced twice a year.
There is much data on wing length for many species from around the world
(see for instance HANZAB or BWP), but I am not aware of any cases where
seasonal variation has been identified as due to moult or the occurence of
two different generations of feathers in one year. Wear of the longest
primary can effect wing length. For example, Oriental Pratincole's
wing-length varies by an average of over 5 mm depending on whether the
primaries are worn or fresh. It could be argued that this wear may be
functionally analogous to that of the Starlings (which attain breeding
plumage through wear of the feathers) only designed to alter wing-shape
rather than appearance. However, I am not aware of any evidence to support
such a view and I for one would take some convincing that it was not just
an inevitable consequence of having a long, skinny outer primary exposed to
the elements all year.
There are some examples of species which show seasonal variation in
tail-shape changes owing to moult. Skuas moult their central tail feathers
twice a year, during both the complete and partial moults. After the
complete (post-breeding or "pre-migratory") moult the streamers are short
and after the partial (pre-breeding or "post-migration") moult they are
long. Some terns, such as Arctic have a similar strategy except that the
streamers are the outer tail feathers. In both these examples the basic
(ancestoral) moult pattern (no. 2 above) has been slightly modified to
include select flight feathers in the partial moult.
There are a few species that moult their flight feathers twice a year. The
only species I know any details of is Franklin's Gull, which is a
long-distance migrant. It does not show seasonal variation in wing-shape.
There are a few examples of ontogenetic change (change with age) of
wing-shape. A simple case is that of falcons, in which juveniles of some
species have slightly longer wings than adults (training wings?!). The most
complex example is that of megapodes. Because they have no parental care
and fend for themselves from day one they are independant at a small size.
They can fly within a few days of hatching, but a newly hatched
Brush-turkey could not raise adult-sized wings; the juvenile wing of a
brush-turkey is proportional to its small size. Megapdoes take several
years to reach adult size, maybe two or three in brush-turkeys. And so they
have an incremental increase in wing and tail size as they moult. In
theory, they can be aged on their size or wing length.
It seems to me that it is physiologically possible for a bird's wing-shape
to change seasonally due to moult. However, it does not seem to happen very
often, if at all. Perhaps, with all the compromises in producing a finely
honed flying feather ball (body shape and size, wing-musculature and
weight, leg and neck length...), altering flight-performance by seasonally
adjusting wing-shape doesn't work? Perhaps the hormonal triggering of moult
(stimulated by seasonal change in the environment or whatever) can't be
relied upon when it comes to aerodynamics? Perhaps I'll stop speculating?
David James
PO BOX 5225
Townsville Mail Centre 4810
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