In American Buddhism, rebirth is not transmigration. There is no self that is being reborn. On the other hand, rebirth in American Buddhism is part and parcel of the continuous process of change, not just in our death and birth. Rebirth is actually manifested at every moment. When we examined our body, though and actions, our experiences are constantly characterized by birth, death and rebirth.
There are various realms, spheres and dimensions of existence. The 6 realms of existence is a common presentation. It can be further divided into 2 groups of fortunate and unfortunate existences.
(1) Realm of Gods (fortunate)
(2) Realms of Demigods (fortunate)
(3) Realm of Human Beings (fortunate)
(4) Realm of Animals (unfortunate)
(5) Realm of Hungry Ghosts (unfortunate)
(6) Realm of Hell Beings (unfortunate)
Among the 3 unfortunate realms, the cause of rebirth in the animal realm is heedless pursuit of sensual desires that resulted in ignorance. The cause of rebirth in the hungry ghost realm is attachment and greed that resulted in miserliness. The cause of rebirth in the Hell realm is repeated violent behavior such as cruelty and killing.
Of the other 3 fortunate realms, the cause of rebirth in the demigods is generosity. However, they are constantly at war due to their suffering from jealousy and envy. The cause of rebirth as Gods is good conduct and meditation. However, they suffer from overindulgence and pride that inhibits their practice. The human realm is the most preferred of the 6 realms because as a human, one has both the motivation and opportunity to practice the Dharma and to achieve enlightenment.