That's a failure of the joke not to set it up -- one of the "top corners" of the square is missing. Chickens normally don't make an effort to get to the "other side" (as far as we're aware anyway).

To make the square you'd have to do something where the context of "the other side" means past life into death. e.g., "Why did the spiritualist put his ear towards the road? To hear from the other side."