The Mocking-Bird

One day in the early autumn Private Grayrock of the Federal Army is rested at the root of a great pine tree. It looked like he fell asleep under the tree but, he was in a frame of mind unfavorable to repose. He was tired because of during the last night he was a sentinel at the post. That night was dark and he was left at his lonely post in a middle of a forest. He stood and stared into a darkness at his front and tried to recognize some acquainted objects. Suddenly he saw a vague figure of a man...
What happened next? Read the story till the end to know.