Animal migration
It is found in all major animal groups, including birds, mammals, fish, reptiles, amphibians, insects, and crustaceans.
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.
[5] One of the most commonly used definitions, proposed by the zoologist J. S. Kennedy[6] is Migratory behavior is persistent and straightened-out movement effected by the animal's own locomotory exertions or by its active embarkation on a vehicle.
[5] Irregular (non-cyclical) migrations such as irruptions can occur under pressure of famine, overpopulation of a locality, or some more obscure influence.
Some species such as Pacific salmon migrate to reproduce; every year, they swim upstream to mate and then return to the ocean.
Organisms like crabs, nematodes, and small fish move in and out of these areas as the tides rise and fall, typically about every twelve hours.
Typically, during low tide, smaller or younger species will emerge to forage because they can survive in the shallower water and have less chance of being preyed upon.
During high tide, larger species can be found due to the deeper water and nutrient upwelling from the tidal movements.
[17] Many of these migrations are north-south, with species feeding and breeding in high northern latitudes in the summer and moving some hundreds of kilometres south for the winter.
[22] Most fish species are relatively limited in their movements, remaining in a single geographical area and making short migrations to overwinter, to spawn, or to feed.
[25] In the 'sardine run', billions of Southern African pilchard Sardinops sagax spawn in the cold waters of the Agulhas Bank and move northward along the east coast of South Africa between May and July.
[27] Exceptionally, swarms of the desert locust, Schistocerca gregaria, flew westwards across the Atlantic Ocean for 4,500 kilometres (2,800 mi) during October 1988, using air currents in the Inter-Tropical Convergence Zone.
[36] Migration is important in cetaceans, including whales, dolphins and porpoises; some species travel long distances between their feeding and their breeding areas.
[41] Some crustaceans migrate, such as the largely-terrestrial Christmas Island red crab, which moves en masse each year by the millions.
Like other crabs, they breathe using gills, which must remain wet, so they avoid direct sunlight, digging burrows to shelter from the sun.
An alternative is the Argos Doppler tag, also called a 'Platform Transmitter Terminal' (PTT), which sends regularly to the polar-orbiting Argos satellites; using Doppler shift, the animal's location can be estimated, relatively roughly compared to GPS, but at a lower cost and weight.
[47]Before animal migration was understood, various folklore and erroneous explanations were formulated to account for the disappearance or sudden arrival of birds in an area.
[49] Another example is the swallow, which was once thought, even by naturalists such as Gilbert White, to hibernate either underwater, buried in muddy riverbanks, or in hollow trees.
