I found this to be an excellent read. It's very long but also very informative.

Avian Anatomy

So many things I don't know. This being one of them. Kita had this bare patch the last time she sat on eggs and I thought she was having some type of nutritional deficiency when I noticed it.

Another adaptation with avian skin is the brood patch, an area over the breast that becomes thickened, very vascular and the feathers are lost during the brooding period. These modifications promote the transfer of heat from the hen to her eggs.

Wow, this is quite an article. I confess I didn't read it all but this paragrapgh caught my eye since Spice has been dealing with sinus infections.

Inside the bones of the bird head are the sinuses and concha. These are hollow spaces normally, but with infection, they may become clogged with liquid, mucus, abscess material or debris. One sinus is found behind the eye, which is why some birds with respiratory and sinus infection may develop swelling and discharge from the eye.

I didn't realize that about the eye and is something I will watch for. Thank you for posting.