w birds were sitting on the fence. x more storks and y more birds came to join them. How many more storks than birds are sitting on the fence?
There were w birds sitting on the fence. x more storks and y more birds came to join them. So, now there are w + x + y birds sitting on the fence. Out of them, w + y are birds, and x are storks.

Therefore, the answer is x - (w + y) = x - w - y.