Stolen from Mad Magazine...
I remember this bit specifically from an issue back in 1993... it was also in a much older issue I found in my grandfathers collection from God knows when.
Difference was that it was a man with popcorn. He feeds the birds and then another man sitting beside him asks for some and takes a bite. Then the birds start to die and the man looks at the bag and sees it says arsenic on it.
Even when I read it in Mad Magazine I found it unoriginal and boring. They probably stole it as well... but as they have the oldest publishing of it (the copy my grandfather owned), I'll be giving them credit.