Yep, and it's fun watching them use it and grooming themselves....dang, they have the most amazing dexterity of the neck!
I remember when my DH and I first noticed this on our chickens and were like "WTH??!", but we knew it was probably no coincidence that they all had it and rubbed it while preening. Once we looked it up it was like "Huh, handy!" Once they had fully feathered out, it always looked like they were opening a lid of feathers on their back when they'd expose it during their preening. To this day we still jokingly refer to it as "opening the trunk".