In Chapter 4, both Billy Pilgrim and his guards are shown to coo at varioius stages and for different reasons. Firstly, Billy Pilgrim "gurgled and cooed" when he experiences a jump in time and goes back to being a baby with his mother. Secondly his guards "cooed calmingly" as they peer into the boxcar where Billy Pilgrim is and usher him towards the camp. The descriptions of these two events, separated by many years, draw a parallel between them. Just as Billy as a baby is carried by his mother and put on the ground, so the guards carry him off the box car as an adult and place him on the floor:
Billy didn't want to drop from the car to the ground. He sincerely believed that he would shatter like glass. So the guards helped him down, cooing still. They set him down facing the train. It was such a dinky train now.
Note how this parallel between these two separate events actually presents time as the Tralfamadoreans see it, with no sense of distance between past, present and future. Billy Pilgrim as a baby undergoes a very similar experience to Billy Pilgrim as an adult, and the use of the word "cooed" to describe the actions of both Billy Pilgrim as a baby and his guards as an adult highlights this narrative perspective. Likewise, both experiences feature Billy Pilgrim's awareness of his own mortality.