Seeing these pictures of bald birds, it reminds me of a bird that comes to visit my feeder, and has been coming since last year. It's a little female house sparrow, with the back of its neck picked completely bare of feathers. It always seems perfectly healthy, apart from that patch of baldness, and even brings little ones with it, to the feeder.
Lately, I have noticed the baldness spreading to the upper parts of its wings, and the sides of its tail. And then, a couple of days ago, I saw another bird land beside it at the feeder, and start picking feathers off its tail, while it ate. It seems like this poor bird has a compulsive mate, which overgrooms it. I had never heard of feather-picking happening in wild bird populations, before, but it seems to be the case, here.
Maybe some of these baldies are also victims of picking.