MIGRATION
Migration is a regular long-distance movement of animals from one place to another and back. Migration is triggered by environmental changes such as day length (most common) or temperature, and may be guided by celestial, magnetic, or olfactory cues. Animals may move north/south or from one elevation to another.

In the 1950s, observers learned that fish that spend their adult lives in the ocean but spawn (reproduce) in fresh water migrate via two mechanisms. In the ocean, they navigate by the angle of the sun, and in fresh water they navigate by smell.

During migration, birds of some species may orient themselves by the sun, stars, horizon, magnetic fields, and/or learning (if they migrate with their parental flock). Hormones from the hypothalamus and pituitary interplay in controlling migration. Some migrations are truly spectacular; a hummingbird weighing a few grams can fly from the southern United States to South American rain forests without stopping!

Invertebrates typically do not migrate far, but the monarch butterfly migrates from Canada and the continental United States to Mexico. Mammals from bats to large ungulates, such as caribou and wildebeest, migrate. Freshwater eels migrate to the Atlantic Ocean to breed. The young eels can migrate back to parenting areas with amazing accuracy. Could you navigate your way back to your family if you'd never been where they are, across thousands of kilometers of open ocean or land?