What counts as migration
Migration is a regular, usually seasonal movement of animals between two or more areas, typically a place for breeding and a place for surviving the off-season. It is different from simply wandering or from a one-way dispersal, such as a young animal leaving to find its own territory. True migrants tend to make the round trip, often along the same general routes year after year.
Examples are found in almost every animal group. Many birds fly between northern breeding areas and warmer wintering areas. Humpback whales feed in cold, productive waters and breed in warmer ones. Wildebeest in East Africa follow the rains and fresh grass. Salmon hatch in freshwater, live in the sea and return to rivers to spawn. Monarch butterflies in North America make a multi-generation round trip, in which the individuals that fly south in fall are not the same ones that flew north in spring.
Why migrate at all
The main reasons are connected to food, safety and reproduction.
- Seasonal food. In temperate and polar regions, summer brings a burst of insects, plants and plankton, but winter does not. Traveling lets animals use the rich season in one place and avoid the lean season there.
- Breeding conditions. Some places are better for raising young, for example with fewer predators, more food for chicks or calm, warm water for calves.
- Weather and climate. Animals can avoid extreme cold, drought or flooding by moving.
Migration is costly. It burns large amounts of energy, exposes animals to predators, storms and exhaustion, and in modern times to hazards such as buildings, power lines and lost habitat. It persists because, in the right circumstances, the benefits outweigh these risks. Where food stays available all year, the same species may stay put: some populations of the same bird species migrate while others nearby do not.
What triggers the journey
Animals do not simply decide to leave when it gets cold. Many rely on internal rhythms that are tuned by outside cues.
Changing day length is one of the best-studied cues for birds. As days shorten or lengthen, hormones change, and the bird becomes restless and builds up fat. Captive migratory birds kept in cages often show agitated hopping and fluttering at the time of year when their wild relatives would be departing, which suggests an inherited internal program. Temperature, food supply, wind and rainfall then help set the exact timing. Climate change has been linked to shifts in the timing of spring arrival in some species, though responses differ widely between species and regions.
How routes are followed
Some routes are largely inherited, and others are learned. Young songbirds that migrate alone at night, with no adults to follow, must rely heavily on inherited information about the direction and approximate distance to fly. By contrast, cranes, geese and many mammals travel in family groups or herds, and young animals appear to learn routes from experienced travelers. Some whales may follow routes taught by their mothers.
Routes often follow geography. Many soaring birds, such as hawks and storks, use rising warm air and avoid long stretches of open water, so they funnel through narrow land bridges. Shorebirds may hop between a small number of key feeding sites, called stopovers, where they refuel. The loss of such a stopover can be a serious problem for an entire flyway even if the rest of the route is intact.
How animals find their way
Navigation is one of the most interesting and least settled areas of animal biology. Researchers have found evidence for several tools, and the picture is still being filled in.
- The Sun. Many animals can use the Sun’s position as a compass, and they need an internal clock to compensate for its movement across the sky.
- Stars. Experiments with night-migrating birds suggest they can use the pattern of stars rotating around the sky’s pole.
- Earth’s magnetic field. Evidence from a variety of animals, including birds, sea turtles and some insects, indicates a sense of magnetic direction or position. How the sense works physically is not settled. One proposal involves light-sensitive molecules in the eye, and another involves tiny magnetic particles. Both ideas are actively researched, and neither should be treated as final.
- Smell and sound. Salmon are widely thought to use the smell of their home stream when they return. Some seabirds may use odor maps over the ocean. Low-frequency sound may help guide some other animals.
- Landmarks. Coastlines, rivers and mountain ranges can help, especially for experienced animals.
It is likely that many animals combine several of these cues, checking one against another. A major open question is how an animal that is displaced to an unfamiliar place can work out where it is, not just which way is north. This “map sense” is demonstrated in some experiments but not fully explained.
Threats and what is being done
Many migratory species depend on several countries’ worth of habitat, so a problem anywhere along the route can affect the whole population. Habitat loss at breeding, stopover or wintering sites, barriers such as dams and fences, artificial light that disorients night migrants, hunting, and shifting climate are all concerns. Some declines in migratory populations have been documented, although trends differ greatly from one species to another. Conservation efforts commonly involve protecting key sites, building fish ladders and wildlife crossings, and international agreements, since the animals themselves ignore borders.
Studying migration has become easier with small tracking tags that use GPS navigation and other technology, and this has already overturned some earlier assumptions about where animals go. There is still much to discover, and that is part of what makes migration so compelling to biologists.