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 sitting on the fence. 
x more storks join them, so the total number of birds and storks sitting on the fence is w + x. 
y more birds join them, so the total number of birds and storks sitting on the fence is w + x + y. 

There are w + x + y birds and storks sitting on the fence in total. 
There are w birds sitting on the fence. 
Therefore, there are w + x + y - w = x + y more storks than birds sitting on the fence.