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 come to join them. 

That means there are now w + x birds sitting on the fence. 

y more birds come to join them. 

That means there are now w + x + y birds sitting on the fence. 

To find out how many more storks than birds are sitting on the fence, we need to subtract the number of birds from the number of storks. In other words, we need to find w + x - (w + x + y). 

w + x - (w + x + y) = w + x - w - x - y = x - y

Therefore, there are x - y more storks than birds sitting on the fence.