From the eBay listing:
When activated, the hatch on top opens, bird appearing, and there is bird song. The bird turns from side to side as it sings. The beak/tailfeathers/wings move to some degree while the bird is singing. The bird darts inside when it has finished singing and the hatch closes automatically.
Here is the full ebay listing with many more pictures of Griesbaum Whistler singing bird box automaton.