Can birds count ?

Bird photographers found that by taking a friend into their hide they could "fool" the birds into believing that the hide was empty once the friend had left. Crippling views ensued as the birds performed brilliantly for the camera [sometimes]. The accepted wisdom to explain this phenomenon was that birds could not count. My own theory is that there is no reason to suppose that birds are unable to count: they have seen one human come and go without causing them any harm and feel safe to assume that the 2nd will do likewise when he is good and ready.