Is this normal for all large birds?

I was wondering if it is normal for a bird to have an almost bald spot under their wings? not really bald, but very short feathers that are more small quills then feathers? Our newly adopted atoo has this and i remember my mothers blue and gold having this as well...it is only under the wings and i can only see it when he opens his wings for preening or when hes goofing off.