What is the fundamental biological reason both vulture types lack feathers on their heads when consuming carrion?

Hygiene, as bare skin is easier to clean and sterilize after feeding inside carcasses

The lack of feathers on the head and neck of scavenging vultures is a critical adaptation directly related to their diet of carrion. When feeding deep inside an animal carcass, their heads become heavily contaminated with blood, fluids, and decaying organic matter. If feathers were present in this area, they would be extremely difficult to clean, creating an ideal environment for the proliferation of dangerous bacteria and parasites. Bare skin allows for effective self-cleaning, and subsequent exposure to direct sunlight provides crucial UV radiation to sterilize the skin surface after feeding.