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 are w birds on the fence. If x more storks and y more birds come to join them, then there will be a total of w + x + y birds on the fence. 

We are looking for the number of storks sitting on the fence. This is equal to x + y. 

Therefore, the answer is x + y.