In some migratory species, the journey for an individual is on a strictly one-way ticket. In Europe, the clouded yellow (Colias croceus), red admiral (Vanessa atalanta) and painted lady (Vanessa cardui) butterflies breed at both ends of their migrations. The individuals that reach Great Britain in the summer breed there, and their offspring fly south in autumn and breed in the Mediterranean region - the offspring of these in turn come north in the following summer.
Most migrations occur seasonally in the life of individuals or of populations. They usually seem to be triggered by some eels and salmon external seasonal phenomenon (e.g. changing day length), and sometimes also by an internal physiological clock. They are often preceded by quite profound physiological changes such as the accumulation of body fat. They represent strategies evolved in environments where seasonal events like rainfall and temperature cycles are reliably repeated from year to year. There is, however, a type of migration that is tactical, forced by events such as overcrowding, and appears to have no cycle or regularity. These are most common in environments where rainfall is not seasonally reliable. The economically disastrous migration plagues of locusts in arid and semiarid regions are the most striking examples.
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