What is "ring-necked blackhead"

Ring-necked \Ring"-necked`\, a. (Zo["o]l.) Having a well defined ring of color around the neck.

Ring-necked duck (Zool.), an American scaup duck ( Aythya collaris). The head, neck, and breast of the adult male are black, and a narrow, but conspicuous, red ring encircles the neck. This ring is absent in the female. Called also ring-neck, ring-necked blackhead, ringbill, tufted duck, and black jack.