Why is walking difficult for stiff-tailed ducks?

Stiff-tailed ducks, which belong to the tribe Oxyurini, find walking difficult because of their specialized anatomy. In ducks, the legs are positioned rearward and set farther apart than in other waterfowl, which naturally limits their movement on land to a waddle.1

This physical development reaches its extreme in the stifftails, whose legs are situated so far back on the body that they can scarcely get about on land. Consequently, these birds rarely leave the water on foot and typically sleep on the water. Their bodies are highly adapted for an aquatic existence, utilizing their specialized, spiky tail feathers for steering while diving underwater in search of food.21