Something weird is going on with the Northern Cardinals at our feeder. First a female missing a leg, now a male minus his head feathers:
A view from the other side:
A bit of searching with the obvious keywords produced that writeup, which suggests feather mites or other parasites. Given that this was in March, that cardinal is definitely not molting!
Those pictures are tight crops from a hand-held Canon SX230HS at dusk, through two layers of 1950-vintage glass. Sorry about that, but the bird spooks whenever I crack the door open for a better view.
The Smell of Molten Projects in the Morning 
Pesticides Ed, pesticides!
I’d expect chemicals to affect the entire bird, rather than just the head, although itchy mites should be all over, too. Perhaps one’s own head is the easiest part to scratch?
Indeed, a mystery…
Maybe the bird should take some antifungal drug?
My father said when his Army company returned to the US from combat in the Pacific, they walked through a “shower hall” that covered them with DDT powder.
It killed off all their many parasites and certainly made sense at the time…