A diving duck, the Ring-Necked duck has a short crest that creates a distinctive peak at the back of the crown – this unique head shape distinguishes this species from similar diving ducks. Males have a black head, breast, back and tail area while their head may show a purple sheen. The slate bill is outlined in white at the base, and has a white band and a black tip. A white wedge separates the breast from the sides, which are finely barred gray and their belly is white – this creates contrast between the light gray flanks and the near black back. In the summer and early fall, adult males have a duller overall appearance with minimal head shine, dingy gray-brown flanks, a dirty coloring on the back and breast with a less patterned bill.
Females are gray-brown overall with a whitish belly while their crown is dark, pale gray cheeks and faded white near the base of the bill. They have a white eye ring and sometimes a pale line extending back from the eye. Their bill pattern is similar to that of the male but lacks the white border at the base.
Distinguishing this species from similar lesser and greater scaup is the very squared off hindcrown and highly patterned bill. Scaup are diving ducks with a black head that shines purple or green. Males of both scaup species are generally paler and closer to white on the flanks. Females and immature scaup lack the white eye ring and the crown to cheek contrast shown by female and immature ring-necked ducks.
Tufted ducks with gleaming white sides are adult males and have a more rounded hindcrown and typically display unique elongated feathers extending off the hindcrown. Female and immature tufted ducks have dark backs and are more suggestive of scaup in that they lack the white eyering and the distinctive angular head shape.