Question: do we know why blackshoulder peahens look Like That? When I saw one for the first time I thought it was a pied bird! Why are they so white? Is BS some kind of lack of pigment on parts that just goes "hidden" in peacocks because the iridescence conceals it?
So the short answer is the bird isn't actually white, she's cream, and also I don't know the exact reason they're like that.
The slightly longer answer is that the solid wing mutation affects the barring pattern on feathers in some manner to cause the barring on males to be replaced by solid color. Barring refers to the striping on individual feathers that forms "bars" of color horizontally over the feather. Whether BS marbleizes them or removes them is unclear, but it alters that pattern type. If I had to make some kind of guess, I would guess that it removes any pigment used in brown/pale brown barring. The "bars" that are added to a peacock's wings are likely the pale stripes, not the black ones, and we can guess this because the wild type greens have solid black wings, not barred.
On blue males, only the wings are barred as adults, and their barring is thick compared to a hen's. And yes, the hens ARE barred, it's just a different kind of barring.
a normal male's wing barring looks like this:
compared to the wing barring on a hen which can be "thicker" in the sense that you can see the barring better, but mostly the barring looks like it does at the top of the photo (the barring in this case is due to Bug's minor amounts of green blood).
But the hens are not only barred on their wings. They are barred everywhere. It's just the barring is very, very fine, and looks kind of like static:
And what happens when you remove all the lighter, non-black (and non-red) pigments from that extra-fine, static barring?
In a bird like a purple, who is colored w/ melanin that's diluted through other factor is you still get a "brown" color, but without actually using those pigments.
Interestingly enough, the actual tail feathers of a BS hen are unaffected by this gene, as they have no barring. Looking at the color of the tail feathers is one of the ways people tell the colors of BS hens apart, when it might otherwise be nearly impossible to do.
We also know they're fully capable of producing red pigments still, AND that the "white" isn't white because when you get certain mixtures of green blood in there, you end up with red buff (BS, but with the reds still in the feathers cranked to 11)
It's ALSO true that adding green blood (which allows hens to sometimes retain barring that is more like the males, but from the green blood so again not the same) means sometimes Spalding BS hens will have the black part of the barring, but not the lighter parts (suggesting maybe the difference is the "bars" on their feathers are the black part, and the base color is cream):
and Spalding BS may look HIGHLY colored to the point they're mistaken for young BS males. This IS a hen- and she's displaying what BS looks like on a hybrid with enough green blood that if she were not BS, she'd likely be barred, or at least have a LOT more thick barring than a blue hen.
The BS mutation also reminds me HEAVILY of the red breasted mutation in button quail:
and I would guess it could be similar. BUT again this is all just speculation on my part, given what I have seen of the gene and how it operates in practice. It would be nice to know the scientific truth some day, but I fear that may not come to light until after my time, if at all.
