As one would expect, the birds are startled every time a shot rings out. Their fright is likened to what would happen to an audience if a musician suddenly struck an incorrect note and ruined a performance. It is a jarring sound in the midst of beauty. In other words, the gunshots are extremely out of place. Were it not for the fact that the Indians and Pakistanis are waging war, this environment would be very peaceful.
In the blue skies, cotton clouds floated all day like barges on a lake.
This creates an image of calmness and serenity, and the reader is not surprised that the birds "[get] startled and [take] flight" when a shot rings out.
The idyllic setting becomes even more peaceful when the writer reveals that the soldiers are weary of this "indecisive war" in which nothing ever seems to happen. It is a dog, however, rather than the birds, who would have done well to be startled and stay far away from the soldiers.
The dog, which is described as an "ordinary looking mongrel," shows up in the Indian soldiers' camp one night and plays a far more significant role in the story than the birds do. The dividing lines have been drawn, and even dogs are required to be classified as either Indian or Pakistani. As the dog's eventual death proves, the war has a far greater impact on him than it does on the birds.