Poo-tee-weet??

The most obvious meaning of this is a bird chirping. What does that mean figuratively? Why would Vonnegut choose to end his novel this way?
After some analyzing of the entire novel and its themes, I reached one conclusion that might or might not be correct about what Poo-tee-weet means.
One of the things questioned in this novel is human nature. Why so much war and destruction? The satirical comments of Kilgore Trout about humanity caring more about someone's breath than if they kill dozens of people is another example of Vonnegut questioning human nature. Just as humans will always fight wars, then, birds will always chirp. Birds will always say, "Poo-tee-weet." However, why is it a question. Is the bird itself questioning human nature. Vonnegut doesn't say that Billy saw a bird chirping. Instead, he says, "One bird said to Billy Pilgrim, 'Poo-tee-weet?' " (Vonnegut 215) The bird specifically says it to Billy. Although I am no expert, I think I can say with some security that this was Vonnegut's meaning:
He has questioned human nature because of Dresden to the point where even birds begin to question it. They don't understand so much destruction and death either.