Dennis is transforming into a cockatoo while begging Amanda, “Won’t you be mine?”
This is done using two exposures per pose: One with Rodman’s head, and one with the cockatoo skull. Then the two exposure layers are blended together in post, in varying proportions. Did you ever see the 1980 sci-fi movie Altered States?
This was trickier than it sounds because the puppet body and its wings are inside a cage. So it helped to keep the neck joint as loose as possible. The swaps had to be done without moving Amanda and without Rodman falling on his broken leg. If either of those things happened, the pair of frames had to be reposed and reshot.