At 20:47 11/05/98 +1000, Stephenyou wrote:
>I must point out
>that moulting is tied more to the breeding cycle than Danny or David
>may believe. Feather moult and the growth of new feathers is associated
>with increased blood levels of thyroxine, a thyroid hormone... and falling
levels of gonadal (reproductive)
>hormones...
Stephen,
I do not deny any tie between moult and breeding. In fact I indicated that
they are both part of a birds cycle. There is nothing inconnsistent with:
(1) birds do not moult because they breed.
(2) breeding (sometimes) triggers moult.
I have no expertise in the physiological basis for moult, and have no
reason to disbeleive that the proximal cause of moult can be tied to
changes in hormone levels associated with the breeding component of the
cycle. However, breeding is not in itself a reason to moult. In birds that
breed once and moult once or twice a year in fixed successive seasons,
change in hormone levels associated with breeding would seem an obvious
stimulus for the onset of moult. In this case it could be adaptive to tie
moult and breeding together, as they always fall in the same relative place
in the cycle.
However, if breeding were to occur irregularly, at intervals greater than
or less than the optimum intervals for moulting then it would seem
maladaptive to tie breeding and moult together. As Danny rightly pointed
out, moulting demands a lot of energy and involves considerable risk. In
such circumstances, hormonal change associated with breeding would be a
disfunctional and inappropriate trigger for the onset of moult.
Breeding as often as possible has some obvious advantages if you can manage
it (ask any Cane Toad), whereas I am not convinced the same is true of
moulting. Why would a bird moult its flight feathers twice in a year (and
waste all that energy) when one set of feathers a year is sufficient?
In the literature it has too-often been assumed that moulting is of
secondary importance to breeding. But consider that a bird that does not
breed this year may live to breed next year, whereas a bird that does not
moult this year will die. For its genes to survive a bird must moult every
year but it does not have to breed every year. For its genes to survive it
is best to breed as frequently as possible but to moult as infrequently as
possible (secondary selection for colour change aside).
You say that Spinnifexbirds were undergoing moult of flight feathers in
July and in Feb. Thus both moults are complete, which would rule out
explanation (2). Explanation (1) accounts for time, but does not account
for energy. Explanation (3) seems like a default answer. I bet that
Spinnifex birds normally have two moults of flight feathers per year,
irrespective of how often they breed.
References
James, D. 1998. Pure speculation and conjecture. Dangerous Publications,
The Back-blocks, Downunder.
David James
PO BOX 5225
Townsville Mail Centre 4810
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