Why do diving ducks have legs placed far back on their bodies?

Diving ducks have legs positioned far back on their bodies as a specialized adaptation for aquatic locomotion. This anatomical placement is highly efficient for swimming and diving, as it allows the birds to propel themselves effectively underwater using their powerful, webbed feet.12

While this rearward leg position is advantageous for life in the water, it is less suited for terrestrial movement. Because their legs are situated far back and set wide apart, these ducks are typically clumsy on land, where their gait is limited to a waddle. In many species, such as pochards and mergansers, this adaptation is so pronounced that the birds carry their bodies in an almost upright position when out of the water. This development reaches its extreme in stifftails, which are so highly specialized for an aquatic existence that they rarely leave the water on foot.31