What in the world is that "cup" on his back for

I acquired this little bird/duck recently, he was peeking out at me from out from a pile of other stuff. Anyway, he captured me right away. The bird is about 18inches long and 14 to the top of his beak. It looks like it is carved out of solid hunk of wood - maybe driftwood. It has some (not a lot) of detailed painting for feathers, red mouth and eyes and has a lacquer finish. I am curious if anyone knows what in the world that "cup-like" indentation would be on his back. It could be an ashtray but it is a bit small (2-3" diameter) and shallow for that. If anyone knows anything about this (age, origin, etc.), please share. For now, we will just stare at one another. Thanks!