Mou thinks that similar genetic tweaks have happened time and again in the evolution of birds. Many groups have lost their neck feathers independently, including vultures, the marabou stork, and large flightless birds like ostriches and emus. Naked necks allow vultures to stuff their heads into carcasses without soiling any feathers; in other cases, a naked neck probably helps its owner to keep cool in hot climates.
Whatever the benefit, it seems that it’s particularly easy for birds to evolve a naked neck, rather than another part of their body. After all, Mou found that the necks of embryonic ducks, turkeys, quails and guinea fowl all have much higher levels of retinoic acid than the rest of the body. This pattern would normally be innocuous, completely hidden from natural selection. But it allows BMP-boosting mutations to denude the neck in one fell swoop, while keeping the rest of the body covered in feathers. As Mou writes, “An underlying map within the skin provides a one-step route to a bare neck.”
The post goes into detail about how the genes initiate the production of chemical activators and inhibitors, and ends with a parable from Alan Turing that explains the concept in layman's terms. Link
(Image credit: Demontux)