The bird sculptor must know and understand bird anatomy and how a bird is structured.
Birds, like mammals (including humans) are vertebrates.
Vertebrates simply means that the animal has a backbone.
Nature has designed just one basic pattern, with many variations, for all vertebrates.
The skeleton of the bird's wing is more easily understood when compared to the human arm.
Comparison between human arm above and bird wing below
Wing drawings, copyright Sandy Scott, from the book Spirit of the Wild Things - The Art of Sandy Scott