Two parrots are sitting on a perch...
Two parrots are sitting on a perch. One turns to the other and asks, "do you smell fish?"
This is a pun, as well as a garden path statement - the person hearing this is supposed to assume that the parrots are sitting on a bird perch - a thing for a bird to perch on, however they are apparently sitting on a perch fish.
The moment when the listener recognizes this, their confusion turns to understanding, and the humor is derived.