Indians For Guns

Two guys go shooting and when a bird takes off, one shouts "Pheasant!" and shoots. "No -says the other hunter- it's a partridge!"
"Let me see" says the other, disappearing in the forest.
After a while, the remaining hunter asks "Partridge or pheasant?"
"According to the ID it's a bank clerk!"

I'm not that familiar with English, could you please explain the joke for me? Because I don't really get it. Thanks