When my birds lift and spread their wings, I can see that some have feathered, normal looking underarms (lets call it this way) while other have no feathers there and their under-wing skin looks red and almost bloody. Is this normal, or some kind of pathology and deficiency?

Hmmmm sorry can't answer that one

The feathering is thin in that area, and if they're molting it might look naked. Keep an eye on them to make sure that they aren't plucking themselves under the wings. That can be a sign of giardia infection.

thank you.
I don't see them plucking but for example Trillie has had it as long as I've known him... Now I see that also Stella has the same looking underwings

I found self - testing kit. what do you think?
http://www.birdsafestore.com/Giardia-Te ... p_645.html

do you know if drinking tap water in US can be contaminated?

I believe that years ago I may have read that it is but don't quote me on it. I had a cockatiel that would overpreen his underwing area and it was a lot thinner than anywhere else

Giardia testing is difficult because the birds don't always shed the organism in their droppings. Even vets can have a hard time getting an accurate result. If you don't see the birds plucking then it probably isn't giardia. It's more likely that the feathers are naturally thin in this area.

The city water supply should be pretty safe because it's tested regularly. But if you have a private well and don't test it regularly there could be problems.

I see birds preening and cleaning feathers, and for me, its hard to say if they clean or pluck underneath their wings (all I can see is that head is being under the wing LOL)
Trillie has always liked to help other birds clean (he is a father, after all), I can hear their Ouch! sometimes when he does it

I give my birds spring bottled water but I was wondering in general. I filter tap water for humans and wild birds by my house get tap water in bird bath