The birds in Slaughterhouse-Five make the sound “Poo-tee-weet”—something that is heard after a massacre. The sound “Poo-tee-weet” is a stand-in, a nonsensical noise made by birds that represents the fact that there is nothing intelligible that can be said about war or massacres. The death and loss from war is not something that can be analyzed or effectively eulogized; the only thing we can say is utterly insignificant in the face of the devastation.

The book focuses on the fire-bombing of Dresden, a real event that Vonnegut lived through as a POW—just like Billy Pilgrim. Vonnegut has Pilgrim go through the same experience so that he can make the statement that the loss and damage of war are ineffable—no words can adequately describe it, and there is nothing we can say to capture the scope or depth of the loss. Vonnegut, in “Dresden Revisited,” goes so far as to say,

The Dresden atrocity, tremendously expensive and meticulously planned, was so meaningless, finally, that only one person on the entire planet got any benefit from it. I am that person. I wrote this book, which earned a lot of money for me and made my reputation, such as it is. One way or another, I got two or three dollars for every person killed. Some business I'm in.

The meaninglessness of the massive loss of human life is encapsulated in the phrase “poo-tee-weet”—and the unnecessary nature of war, in general, is what Vonnegut captures by closing his novel by the words. Ultimately he leaves the reader with that question—“Poo-tee-weet?”—asked by the birds, because really, what else is there to say about the devastation? Nothing anyone can say will ever really mean any more than the sound of a bird chirping in the silence of so many dead.

In the first chapter, when discussing the difficulty that he had writing the anti-war novel, Vonnegut explains that "there is nothing intelligent to say about a massacre." Since the bombing of Dresden is a massacre, there is nothing intelligent to say about it. Since the last scene of the novel occurs shortly after the bombing of Dresden, Vonnegut ends with a nonsensical and unintelligible question: "Poo-tee-weet?"

By ending the novel this way (by asking a question that makes no sense), Vonnegut drives home the point that war makes no sense and that the bombing of Dresden was a senseless act. Furthermore, "Poo-tee-weet?" is a question and not a statement of fact. Thus, Vonnegut ends with a nonsensical question that the reader cannot answer intelligibly, and the reader is in a similar situation as Vonnegut, who can find "nothing intelligent to say about a massacre."

Finally, as Vonnegut states in the opening chapter, it is the birds that ask this closing question.

"Everything is supposed to be very quiet after a massacre, and it always is, except for the birds. And what do the birds say? All there is to say about a massacre, things like "'Poo-tee-weet?'"

Humans are the only animals capable of such destruction, and the birds are left to question this horrific act. Throughout the novel, Vonnegut shows that animals are innocent victims of war as seen in the wounded horse that Billy weeps for and in "Princess," the dog with its tale between its legs, who has been dragged into the war.

One of the main themes of Kurt Vonnegut's Slaughterhouse-Five is the destructive nature of war. The book examines the anachronistic experiences of Billy Pilgrim, a veteran and survivor of the Dresden firebombing who has "come unstuck in time."

Billy first hears the "Poo-tee-weet" from a bird singing outside his window when he is in the ward for nonviolent mental patients in a VA hospital in New York. This bird makes a reappearance at the end of the novel; World War Two has just ended, people are flooding into the streets, and the "[b]irds were talking." Billy hears this phrase once again as the last line of the book.

Earlier, we get context for what "Poo-tee-weet" means:

...[T]here is nothing intelligent to say about a massacre. Everybody is supposed to be dead, to never say anything or want anything ever again. Everything is supposed to be very quiet after a massacre, and it always is, except for the birds. And what do the birds say? All there is to say about a massacre, things like Poo-tee-weet?

So, Vonnegut chooses to end the book with "Poo-tee-weet?" in order to allude to the uselessness of commenting on something as horrific as a war. "Poo-tee-weet" effectively means nothing; to end the book with a meaningless statement, an answerless question, echoes our inability to account for the devastation of war.