How did the chests of chickens come to be known as breasts?

Since chickens aren't mammals, their "breasts" are not mammary glands. However, "breast" can also mean just the area of the body. Chicken breasts are also known as "white meat" since the flesh is lighter that than of the legs (the "dark meat"). Birds' breast muscle is what powers their wings, although in domesticated fowl such as chickens and turkeys, its bulk prevents the birds from flying.