In my reading, I found this quote taken from a letter from a young British lieutenant serving in the trenches in France.
“On April 18, the 22-year-old Lieutenant Robert Sterling, who at Oxford a year earlier had won the Newdigate Prize for his poetry, wrote to a friend of how, some three weeks earlier, he was in the trenches as the Germans were shelling them, and suddenly saw a pair of thrushes building a nest a few yards behind his line. ‘At the same time, a lark began to sing in the sky above the German trenches’, Sterling wrote. ‘It seemed almost incredible at the time, but now, whenever I think of those nest-builders and that all but “sightless song”, they seem to represent in some degree the very essence of the Normal and Unchangeable Universe carrying unhindered and careless amid the corpses and the bullets and the madness.'”
Gilbert, Martin. The First World War, A Complete History. New York: Henry Holt and Company, Inc., 1994, 143.
There are several moments in the play where we see just this sort of moment: a wild charge where suddenly Charlie is drawn to a single calm and lovely detail that reminds him of home; a wild race on horseback where time seems to stop for a moment and Mary sees a single isolated bird in tiny and complete detail. For me, these moments represent the soul of the play – the moments when the characters connect with the world around them in a way that is very personal and completely at odds with the specifics of their actual surroundings.