[NatureNS] Re:Feather pigments - a partially aneumelanistic Blue Jay

Date: Sun, 10 Jan 2010 17:15:10 -0400
From: iamclar@DAL.CA
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Quoting Eric Mills:

"To save a lot of memory work, I suggest "pigmentally-challenged"
(politically-correct, and maybe attention-getting too) for anomalously 
coloured
birds, followed by a short description of what they look like."

Hah, friend Eric, I agree that the terminology is arcane and unnecessary in
everyday parlance, but I do like to tease. Also, when trying to unravel 
CAUSES,
it's sometimes useful to use precise terms. Take this Blue Jay currently at
feeders of Fern McCuish in Hammonds Plains, HRM, photo'd by her recemntly, and
the photo forwarded to me by Ken McKenna. You can see this bird under "other
passerines" in NS-RBA photos if you're signed up.

The bird has white (not normal gray) flanks and a lovely pale blue mantle and
head, with no black markings on the head. But the feathers are wispy 
and frayed
and it doesn't look very sleek at all in those parts. Yet its flight feathers
are all sleek-looking and barred with black, as in normal birds. So, it's
PARTIALLY ANEUMELANISTIC - so there!

The bird lacks black EUMELANIN in those CONTOUR (body) FEATHERS that normally
have small amounts to darken the white flanks and deepen the blue of the back.
Why is that a problem? Trouble is that feathers that have melanin need it to
help strengthen the feathers against wear - otherwise they they get frayed and
messy, like our jay's (and like the faded wingtips of gulls before they 
molt in
summer). It may be even worse if those feathers  are  STRUCTURALLY COLOURED,
like all blue feathers in birds. These have open spaces in the feathers that
refract and reflect light to produce the colours seen. Melanin particles may
help modify the tone of these colours, and also strengthen the feathers.
Because of their open structure, these feathers are very wimpy without melanin
-hence our messy blue jay. Note on the photo of the Blue Jay that the feathers
along the middle of its breast and belly seem more slick. But they never did
have melanin - nicely white in normal birds - and are presumably constructed
more robustly without depending on melanins.

So, why wasn't our House Sparrow equally messy. Remember that sparrows have no
structural colour (only brown and black pigments), so their feathers 
may not be
as weak as the blue ones of the Blue Jay, and this individual retained its
PHAEOMELANINS that may have given enough strength to its feathers.

Thus endeth the perfectly useless lesson - possibly of mild interest to some
(and really a working out of my own partial understanding of the issue).

Cheers, Ian

Ian McLaren

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