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 originally sitting on the fence. 
Then, x more storks came, so now there are w + x storks on the fence. 
Then, y more birds came, so now there are w + x + y birds on the fence. 

To find out how many more storks than birds are sitting on the fence, we want to find w + x - (w + x + y). This can be simplified to x - y.