w birds were sitting on the fence. x more birds and y more storks came to join them. How many birds are sitting on the fence?
There were w birds sitting on the fence. 

Then, x more birds came to join them. 

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

Then, y more storks came to join them. 

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