Migration
Probably a clearer understanding of migration may be attained if shorter movements are discussed first. Some movements of animals tend to lead them into new territory. Diurnal-nocturnal migrations are often important; insects and some vertebrates migrate from forest to grassland at night, returning to the forest for the day, there are also foraging cruises, as in those of the wolf group already mentioned. Migration proper is concerned most often with a change of position having to do with adverse seasons, reproduction, or some obscure cause as migratory birds. When the organism does not revisit the starting point, its movements may be termed emigration. When a return journey is made by different individuals or at irregular times the migration may be called return migration or remigration. Emigration is usually said to be a consequence of population pressure in relation to the food supply, but evidence for this conclusion is very often wanting and in some instances evidently incorrect as in the emigration of certain grasshoppers. It is again most frequent in ungulates, but occurs also in rodents and carnivores.
A conspicuous example is afforded by the lemming of Europe. This is the most discussed if not the most dramatic of known mammalian emigrations, particularly in relation to cycles. With respect to adverse seasons, regular migrations are most commonly made with reference to altitude or latitude. The best-known examples are among animals with powers of flight, such as birds, bats, and insects, and among the ungulates, but are by no means confined to these groups. Mammalian migrations are well illustrated by the barren-ground caribou and the bison. Current migration appears to be more characteristic of fishes than of any other aquatic group, but emigration is more or less exceptional so far as is known. The former is regularly associated with spawning and hence involves a marked change from the usual habitat of each species, such as a change from salt to fresh water, or the reverse. The European herring passes the spring breeding period, and the subsequent feeding one, in shallow waters and then retires northward into deep water for the quiescent season of winter. The young are said to follow the adults for part of the spring journey for several years, until they too are mature and ready to spawn, thus indicating that instinct is not the guide. Other fish with a more or less similar habit are the mackerel, tarpon, pilchard, hake, garfish, bluefish, the sharks, and perhaps the cod. The number of species known to move from coastal to deep waters is much smaller, but among them are the conger eel and swordfish. The behavior of anatropous fishes has attracted much attention. If it is assumed that the bony fishes probably originated in fresh salmon, shad, etc., belong to a fresh-water group, the usual procedure in describing the life history should be reversed, and the migration of the young fishes to the sea regarded as an excursion to an area of rich food supply, much prolonged over the initial visits when the habit began. The return may be to the stream in which they hatched from the egg, a fact that has not quite been demonstrated. Such anatropous fishes as the salmon and shad are characterized by a return migration from the rivers to the sea. It is the return up the rivers at maturity that has attracted attention. The individuals of many species die after spawning. During the long journey upstream, the salmon do not feed and have few important relations with other animals. However, in the lakes in which they breed, the dead bodies of the adults foul the shores and waters, though the effects of this have not yet been investigated. By virtue of a longer life span, the shad and alewife return to the sea after spawning, and hence may repeat the visit to the parent stream several times. Fish is marked by the reverse habit, but it's relatively few in number. The most remarkable example is the eel, the breeding grounds of which lie between the Bermudas and the West Indies. The young migrate from these breeding grounds in the spring, entering the estuaries and rivers of Western Europe and eastern America, where they remain for five to twenty years or more. The behavior of fresh-water fish is like that of marine species to the extent that many of them seek the shallower water of rivers or their tributaries, while a few travel in the opposite direction.
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