At 08:53 AM 8/05/98 +1000, Chris Davey wrote:
>On thinking further about wing shapes whilst shaving this morning (always a
>good time to think!) it occurred to me that I am not aware of any species
>which wing moults more than once a year, unlike body moult, so there is a
>problem and hence why, in the course of a year, you cannot have two
>different wing shapes.
Wing moult is a serious business for a bird: lots of energetic costs related
to growing a lot of feather protein that is relatively rich in a couple of
sulphur-rich amino acids that may be hard to come by; increased water loss;
impaired insulation; impaired flight abilities that increase flight costs
and can make birds much more susceptible to predators. It is also a
time-consuming process: even if a bird is prepared to drop all its flight
feathers simultaneously (thus becoming flightless) wing moult will take
about 3 weeks to a month. More often wing moult is spread over two to four
months, and in big birds (which have to grow more feather material) it can
take much longer. There are lots of good reasons for birds to avoid moulting
their wings more than once a year.
Nevertheless, there are a few birds which replace their primaries and
secondaries twice a year. They seem to fall into two categories: (1) Small
passerines inhabiting thick grass and undergrowth, in which the primaries
abrade very quickly. They include a few Cisticolas and Prinias, and a
Columbian population of Rufous-collared Sparrow Zonotrichia capensis. I
think there are one or two other New World species that have slipped my
memory. (2) A few long-distance migrants: Willow Warbler, Bobolink, and
rather oddly, Franklin's Gull (the only large bird I know of that replaces
all its primaries twice a year).
There might be a few other species that do similar things (there are lots of
birds that have never had their moult studied in detail) but it's fair to
say that double wing-moult strategies are rare. As far as I know, no change
in wing shape from moult to moult occurs in any of the birds listed above.