A bird shows up a couple times in Portal 2. First, it turns out that Wheatley escaped using a bird's egg (not sure how...). Then, a bird (presumably the mother of the eggs) comes and attacks Wheatley, causing him to cry out "AHH! Bird! Bird!"
A while later, a bird starts eating potato-GLaDOS. You eventually find its nest and get it off of GLaDOS, but then you see it later on: "Bird! Bird! Kill it! It's Evil!"
Did Valve mean anything by including the bird? It's the only other animal apart from yourself. Is this the same bird that attacked Wheatley? How did it even get that deep into Aperture Laboratories? Do all robots have Ornithophobia?