(d) : Migration is the seasonal movement of complete populations of animals to a more favourable environment. It is common in mammals (e.g. porpoises), fish (e.g. eels and salmon) and some insects but is most marked in birds. The arctic tern, for example, migrates annually from its breeding ground in the Arctic circle to the Antarctic. Salmon usually migrate from marine to fresh water to spawn. The Siberian crane, breeds in arctic Russia in Yakutia and western Siberia. It is a long distant migrant. Salamander is an amphibian with slender body, short legs and long tail. Outside the breeding season they are seldom seen as they spend most of their time underground.