The Bird Remembers
Poly is a being somewhat unique in SS13 in that while nearly everything else about the game resets after the round ends the bird does not.
Specifically Poly's memories don't reset.
Poly keeps a large list of sentences overheard in game that persists between rounds. What was said in one round can be carried forward to another, free of its original context. He also carries over a few other relevant facts, like how long it's been since someone's killed him out of annoyance (he gets better don't worry).
Poly knows that his world is repeating endlessly, but thankfully is too much of a parrot to be filled with existential dread over this.
Fun with Poly
Poly learns randomly, both from things said in front of him and over the radio. Since Poly is the Chief Engineer's pet and starts every round there, his speech is somewhat biased to that of the CE's. Because Poly starts with a headset people can teach him phrases over the common channel without even realizing it. This is what makes listening to Poly so entertaining.
If you want to enjoy Poly ingame more give him a pet, every time you do he'll spout a kernel of wisdom. Poly also loves crackers, and if you feed them to him he'll grow progressively more and more talkative.
Keep in mind that the chattier Poly becomes, the harder it will be to keep the people who don't appreciate his wisdom from trying to turn Poly into an ex-parrot.