This little fella first pipes up in line 21 of "Four Quartets," telling us to "find them, find them," and probably referring to the children who are playing in the speaker's garden (also check out "Symbols: Playing Children"). Now rather than being some sort of parrot, the bird the speaker is talking about here is probably just your average bird. He just happens to interpret the sound a bird makes as telling us to pay more attention to our immediate surroundings.
Line 21: Here, the bird makes its first appearance, and you don't really know what his deal is. But at this early point in the poem, the speaker is implying that you're looking for something in your life that you haven't really found yet, and the bird is here to remind you of that fact.
Lines 28-29: Here, the bird shows up again, this time pointing you toward some sort of "unheard music" that a better outlook on life might help you to hear.
Line 42: The bird tells us to get more in touch with the type of innocence that allows children to live in the moment, instead of worrying about the past and future.
Lines 44-45: In a strange line, the bird reminds us that even though our goal is to get more in touch with reality, we can probably only handle reality in small doses.