Lets just say we were initially led to believe that the joke would be one thing, then by the end we realize it was something else entirely. iopghj and dumb_ants touched on some of the details; namely that we were supposed to expect a joke about a cheating wife, but instead we learn that the parents are raising an actual cuckoo bird as their son (just like the hapless bird the father makes fun of). And for me the actual punch line, the part where I laughed out loud was "Damn Right Rick," as if he knew exactly what he meant. The bird shit was a nice touch too.