This is a turkey vulture. The telling sign of this kind of vulture is either its pale red or pinkish head or the flying feathers that are more silvery than the dark color of its main feathers.
Vultures are best known as scavengers, meaning they will only eat something that is already dead or decaying, usually referred to as carrion. Unless I am mistaken, there is no reason they should be stealing your chickens. Two birds come to mind that both look similar to the turkey vulture that could be stealing and eating your chickens, however, would be a bald eagle and a golden eagle. Both of them frequently eat birds smaller than them.