Why did the boys set the bird free?

The story "Cranes" is about two boyhood friends that have been placed on opposite sides of a violent conflict. Tok-chae has been captured and Song-sam is escorting him to a new location. As the men walk, Song-sam can't help but ask Tok-chae why he chose the side that he did. Tok-chae gives a simple explanation, and the conversation begins to turn more personal. For example, Song-sam is surprised to learn who Tok-chae married. As the men continue talking, Song-sam's aggression and anger begin to lessen, and he begins to reminisce about being kids together. Eventually the two men reach a hill that overlooks a field. In the middle of the field is a group of cranes. This view sparks a memory about the crane that he and Tok-chae had captured when they were twelve. They kept their captured bird a secret and paid it daily visits.

Once, when Song-sam and Tok-chae were about twelve, they had set a trap here, without the knowledge of the adults, and had caught a crane, a Tanjong crane. They had roped the crane, even its wings, and had paid daily visits, patting its neck and riding on its back.

A few days later, the boys learn that a man from Seoul has a permit to catch cranes as "specimens or something," and the boys are immediately worried about what will happen to their crane. They let the crane go. They probably do this because deep down they know the bird is better off in its natural habitat than it is locked up in a cage to be studied for months on end.