Final answer:
Ducks that run across the water to gain momentum for takeoff are called Dabblers. They have unique feathers that repel water, helping them to float despite their greater density than water due to the principle of surface tension.
Step-by-step explanation:
Ducks that patter along the water's surface for some distance before takeoff are known as Dabblers. Dabbling ducks are waterfowl that typically feed in shallow water by tipping forward and grazing on underwater plants. They use their strong legs and webbed feet to propel themselves across the surface, often needing to run across the water to gain the momentum necessary for takeoff. Unlike divers, they do not typically dive underwater for their food.
The ability of ducks and other waterfowl like geese and swans to float is due to the special properties of their feathers that repel water, maintaining their buoyancy and enabling them to sit on the water's surface. This is also supported by a principle related to surface tension, which allows creatures like water skaters to stay above the water without sinking. Ducks cannot sit on soapy water because soap decreases the water's surface tension, making it more difficult to support the weight of the bird.