Desert locusts once flew 4,500 km across the Atlantic Ocean
In October 1988, swarms of desert locusts rode tropical air currents 4,500 km west across the Atlantic. Arctic terns fly at least 19,000 km a year to enjoy two summers. Migration turns up in every major animal group, yet biologists still lack a single agreed definition of what it is.
Migration usually means individual animals travelling relatively far, typically with the seasons, and it occurs in birds, mammals, fish, reptiles, amphibians, insects and crustaceans. Triggers include climate, food supply, time of year and breeding. To qualify, a movement should recur annually or seasonally, or mark a major life shift, such as young Atlantic salmon or sea lampreys quitting their home river at a few inches long. Before any of this was understood, folklore filled the gap, claiming barnacle geese hatched from goose barnacles.
Zoologist J. S. Kennedy offered a widely used definition built around persistent, straightened-out travel during which an animal temporarily suppresses its urge to stay put. The concept bundles several ideas: long directed movement, relocation beyond daily range, seasonal shuttling between two places, and redistribution within a population. Some species must migrate while others do so optionally; populations may migrate completely, partially, or differentially by age or sex. Irruptions, driven by famine or crowding, are irregular rather than cyclical. Researchers follow travellers with leg rings or electronic trackers.
Timescales vary widely. Many aquatic animals rise and sink a few hundred metres through the water column daily, and some jellyfish drift sideways a similar distance each day. Crabs, nematodes and small fish ride roughly twelve-hour tides in and out of rich intertidal zones, with small foragers favouring low water when predators are fewer. Pacific salmon swim upstream each year to spawn.
Roughly 1,800 bird species, out of some 10,000 worldwide, make long seasonal journeys, timed mainly by day length through hormonal cues and guided by sun compasses and magnetic sensing. Among fish, only a few hundred species travel far, and around 120, including several salmon, move between salt and fresh water. Capelin circle Iceland to feed near Jan Mayen, and in South Africa's sardine run billions of pilchards head north up the east coast between May and July. The globe skimmer dragonfly crosses the ocean between India and Africa, the longest sea passage of any insect.
Source: Animal migration
