Male cardinal feeds sunflower seed to a juvenile that cannot fly to the
feeder-I've never seen anything like this.

Note from FZ10 group bird expert: Your cardinal is feeding a juvenile Brown-headed Cowbird. Cowbirds parasitize other birds' nests, laying eggs and leaving the unsuspecting nest-building parents to care for the cowbird offspring. The adoptive parents will sometimes feed the interloper to the exclusion of their own young.