1. travel by nonhuman animals over relatively long distances to or from breeding areas. Migration is observed in birds, fish, and some mammals and insects (among others). In some species, it is seasonal, involving movement from a breeding area to an overwintering area; in others, particularly the salmon, it is observed only once in the lifetime of an individual. Factors influencing migratory behavior include chemical cues, pituitary or other hormones, relative change in day length, and temperature. See also migratory restlessness; navigation.

2. in the development of the nervous system, the movement of nerve cells from their origin in the ventricular zone to establish distinctive cell populations, such as brain nuclei and layers of the cerebral cortex.