A long time ago, birds used to live in heaven with the spirits. From heaven they would fly down, and interact with the human beings; afterwards, they would return to heaven. At that time, human beings did only when they slept. As they slept, the spirits would come and kill those they wanted to kill.
The human beings then struck a deal with the birds that the birds should always inform them whenever the spirits wanted to kill anyone. The birds agreed to do so and anytime they went to heaven and heard the spirits discussing and planning on the human beings that were to die, the bird would quickly fly down to inform the human beings.
The human being who was proposed by the spirit to die would struggle not to sleep throughout the period the spirits planned for him or her to die. For a long time, no one could be killed by the spirits because of this deal between the birds and human beings. The spirits were worried and sought to know why any human being they planned would die did not sleep.
They later found out that the birds were informing the human beings of anything they planned to do. The spirits chased the birds out of heaven and the birds went to live with the human beings. But the human beings chased them away because they were no longer giving them information about who the spirits wanted to kill. The birds went to live on trees, but kept flying high into the sky to plead with the spirits to accept them back into heaven.