Elitist chicken fanciers, I know, right? And vulture hocks? What the freak?
Chickens have a hock joint, it's right below their thigh. Most chickens have "clean legs", free of feathers. No self-respecting chicken would want to have feathers on their legs, it collects dirt, mud, and makes taking off difficult. Sometimes, a chicken would throw a mutation, feathers on the feet or coming off the hock.
People like the fluffy, lose feathering, as seen on Cochins or Brahmas. For some reason, though, chicken fanciers hate vulture hocks*. These are stiff, almost like flight feathers, protruding from the hock of the chicken.
An example is below, a picture I took of a rooster rescued from a neglect situation. He was one of 140 chickens brought in, with 50 roosters now living beautifully together.
And another in which the hock feathers look more like handsome tuxedo coattails than some freakishly unattractive mutation:
Today, vulture hock is a disqualifying feature in most breeds, with a few exceptions like the Sultan. In fact, some breeders - much like with white Boxers - encourage the culling of birds exhibiting vulture hocks. Unlike the possibility of deafness with white Boxers, vulture hocks in no way medically or behaviorally harm the bird. It's a recessive autosomal (not sex-linked) genetic mutation that purebred chicken fanciers just don't like.
They haven't liked it since it appeared in the early 1800s, for reasons I cannot quite divine. If you peruse old issues of the Journal of Horticulture and Practical Gardening online, you'll find that vulture hock debates often spiraled into insults and verbal vomit. Just google "journal of horticulture and practical gardening vulture hocks" and see for yourself.
You can still find discussion of the trait in modern backyard chicken forums.
While vulture hocks is a recessive trait, its presence is thought to be tied to the feathering on other parts of the legs and feet. And since fanciers want some breeds to have feathering on the hock, it is difficult to eliminate vulture hocks without breeding for birds who have clean legs or ptilopody without shank feathers.
It turns out that it doesn't matter the fancy, so-called purists will always argue over phenotypes that are absolutely and completely useless to the animal in question, like vulture hocks.
I like vulture hocks but don't care whether a bird has them or not. When trying to place the birds pictured above, it's been interesting hearing from chicken fanciers who see no reason for these rescued chickens to exist. They're mutts. Most people unfamiliar with chicken breeds would find these birds pretty, with their feathered feet and different colors and feather variation. I certainly do, but I love them more for their different personalities and quirks than how they look. And that is probably the key difference between me and a diehard breed purist.
*Soft feathers can come from the hock joint and technically vulture hocks are just a "stiffer" version of the preferred hock feathers, but the common connotation of "vulture hock" is stiff feathers, not the softer, shorter and preferred versions.