Text from Khan Academy

Northwest Coast transformation masks manifest transformation, usually an animal changing into a mythical being or one animal becoming another. Masks are worn by dancers during ceremonies, they pull strings to open and move the mask—in effect, animating it. In the Eagle mask shown above from the collection of the American Museum of Natural History, you can see the wooden frame and netting that held the mask on the dancer’s head. When the cords are pulled, the eagle’s face and beak split down the center, and the bottom of the beak opens downwards, giving the impression of a bird spreading its wings (see image below). Transformed, the mask reveals the face of an ancestor.