Like mammals, birds are homeothermic or warm-blooded animals. This means that their body temperature remains stable despite how cold or warm their environment is. To retain a high body temperature even in cold conditions, birds have thick and compact feather layer, an insulating down layer, a thick layer of fat, and an effective circulatory system. In addition to all of these, birds need to maintain their body temperature from energy they receive from their food. 

Waterfowl, such as mallards, geese and swans, have a thick feather layer or plumage. The plumage is so thick and compact that water cannot penetrate it. To maintain the structural rigidity and compactness of their plumage, these birds have preen glands above their tails. The preen gland exudes fat that the bird uses to cover its feathers with. Waterfowl use up a lot of time in the activity of maintaining the condition of their plumage in this way. For them, keeping their plumage well maintained is a matter of life or death.

The feather layer protects a layer of soft, warm feathers called a down layer. The down layer acts as a heat insulant and is kept dry with the help of the protection from the plumage layer. Thanks to these two feather layers, waterfowl can swim in cold water while keeping up their high body temperature. 


The common goldeneye maintaining its plumage.