“Some domestic birds have interesting headgear. Chickens have combs and turkeys have snoods and wattles, but most ducks simply have smooth feathers on their heads (albeit sometimes brightly colored feathers).
Not crested ducks. Not to be confused with the wild crested duck of South America, this fancily-quoiffed duck is descended from the domestic mallard and sports a pouf of feathers growing out of back of its head. This crest is actually caused by a genetic mutation that duck breeders have selected for. This mutation causes a duck to be born with a gap in its skull, which is filled with a growth of fatty tissue. It’s from this growth that the pouf of feathers sprouts."
Information From Animal Planet