I received the following from Win Filewood:
"I've read/heard something similar about Golden & also Rufous Whistler. As I
understand it birds are opposite to mammals in sex chromosome patterns : males
are the equivalent of mammalian females (XX) & females of course the opposite.
I understand that avian geneticists prefer the notation WW/WZ for males/females
to avoid confusion in ignorant theriocentric (mammal-dominated) folk. This is
one explanation why there is an excess of males in many bird species - females
are more likely to have sex-linked disabilities equivalent to human haemophilia
or red-green colour-blindness. However a recent astonishing finding is that
female birds (at least of some species) can actually control the sex of their
offspring to some degree. How, I know not, if anyone yet does.
I can't see how the actual chromosome pattern of a bird could change during its
lifetime. In mammals, both sexes produce the same sex-hormones, just in
different proportions. The Y-chromosome carries very few functional genes but
one is the testosterone-making main switch. However females also produce some
anyway, and there are plenty of "gender-confused" folk out there to show that
proportions not only differ in time in any indivual, but also bewteen
individuals. I suspect that some such mechanism, involving a gradual weakening
of estrogen production, and/or strenthening of male hormone production, is the
cause of masculinization in whistlers over time. If in fact it really happens.
I think it does - we should look at Shrike-thrushes too, but throughout HANZAB
& other plumage description refererences, the phrase
"brighter/more-extensive/&c in OLDER FEMALES" is often used. Yet this might be
circular - brighter females may be assumed (by mammalian chauvinists) to be
older!
Lots of food for thought from a single band-recovery! And people ask me why I
do it."
Greg Clancy
==============================www.birding-aus.org
birding-aus.blogspot.com
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