Northern Cardinal (Cardinalis cardinalis)
I have also seen Cardinals like this on a couple of occasions. I don't know what causes it, but the ones I've seen (or the same bird, twice) seemed perfectly healthy. Now that you've posted this pic, I realize it's more common than I thought. Thanks for posting!
Feather mites. They're mostly around the head where the bird can't preen itself so well. New feathers will grow in with the late summer moult.
Not a serious problem, as in summer it is warm enough for the birds not to need the feather protection so much.
Thanks, Resin! I didn't realize the feathers would grow back. That's encouraging. I just assumed it was something they'd have their entire lives. I've only seen it in Cardinals.