Which of the following is NOT a type of duck?

The green-breasted merganser is actually a species of merganser, which belongs to the family Anatidae but is categorized separately from true ducks. True ducks are typically part of the subfamily Anatinae. The other species listed—mallard, pintail, and wood duck—are all classified as true ducks within the Anatidae family and specifically fall under the subfamily Anatinae.

Mergansers, including the green-breasted merganser, are known for their unique adaptations that allow them to efficiently catch fish, including a long, slender body and a serrated edge on their bills. While they share the same general family as ducks, they represent a different ecological niche and behavioral adaptations compared to true ducks. Recognizing these distinctions is crucial for understanding avian classification and ecology.