Migration (ecology)

[1] Species migrate to take advantage of more favorable conditions with respect to food availability, safety from predation, mating opportunity, or other environmental factors.

[4]: 69–70 The factors that determine migration methods are variable due to the inconsistency of major seasonal changes and events.

This indirectly helps determine an organism's fitness by increasing the likelihood of its survival and reproductive success.

[5] It is found in all major animal groups, including birds,[6] mammals,[7] fish,[8][9] reptiles,[10] amphibians, insects,[11] and crustaceans.

[5] To be counted as a true migration, and not just a local dispersal or irruption, the movement of the animals should be an annual or seasonal occurrence, or a major habitat change as part of their life.

A major habitat change could include young Atlantic salmon or sea lamprey leaving the river of their birth when they have reached a few inches in size.