What is migration

Immigration is the process of moving to a new country or region with the intention of staying and living there. People may choose to immigrate for a variety of reasons, such as employment opportunities, to escape a violent conflict, environmental factors, educational purposes, or to reunite with family. The process of immigrating to the United States can be complicated and is often driven by a few key principles including uniting families, boosting the economy with skilled professionals, promoting diversity, and helping refugees.

Learn more about U. Students discuss why some species migrate. Then they analyze specific examples of migratory species, learn about types of animal migration, and match various animals to their types of migration. Join our community of educators and receive the latest information on National Geographic's resources for you and your students. Skip to content. Photograph by James L. Twitter Facebook Pinterest Google Classroom.

Then tell students that people move for many reasons, and that types of human migration include: internal migration: moving within a state, country, or continent external migration: moving to a different state, country, or continent emigration: leaving one country to move to another immigration: moving into a new country return migration: moving back to where you came from seasonal migration: moving with each season or in response to labor or climate conditions 2.

Tell students that people who migrate fall into several categories: An emigrant is a person who is leaving one country to live in another. An immigrant is a person who is entering a country from another to make a new home. A refugee is a person who has moved to a new country because of a problem in their former home.

Learning Objectives Students will: list and explain main types of migration describe categories of people who migrate list reasons for migrating. Teaching Approach Learning-for-use. Teaching Methods Brainstorming Discussions. Resources Provided The resources are also available at the top of the page. Physical Space Classroom Grouping Large-group instruction.

Background Information Human migration is the movement of people from one place in the world to another. Prior Knowledge None. Vocabulary emigrant. Media Credits The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit.

Nevertheless, migration is a single phenomenon, and we should aim to recognize its unitary structure. The model incorporates both components and processes the changes in and linkages between components and explicitly considers the environment in which the migrant population survives as well as the migrants' responses and adaptations to it.

The four system components are as follows: 1 A migration arena comprising the environment including biotic elements to which the migrants are adapted; 2 a migration syndrome, which is the suite of traits enabling migratory activity—this suite comprising both locomotory capabilities and a set of responses or nonresponses to environmental cues that schedule and steer the locomotory activity; 3 the genetic complex that underlies the syndrome; and 4 a population trajectory or its long-term average, the population pathway comprising the route followed by the migrants, the timing of travel along it, the points along it where migration temporarily ceases, and the times when these points are occupied for breeding and other key life stages.

The model incorporates the ultimate selective and proximate factors acting on migration the arena , the response to natural selection in the phenotype and genotype of migrants the syndrome and its genetic complex , and the population consequences in terms of both selection and current conditions the trajectory and pathway. Its aim was to identify and describe qualitatively how a migratory adaptation functions by employing movement to exploit a changing and spatially extensive environment, and how a capacity for appropriate movements is maintained within a population.

A demonstration of the model was the examination of migratory adaptation in a relatively well-researched moth species, the oriental armyworm Mythimna separata Drake and Gatehouse Gauthreaux provides both a comprehensive list of the geophysical factors, short- and long-term, that contribute to the migration arena and numerous examples of migrant species and their arena-specific adaptations. Although the outcome of migration can be viewed as a population process, it is useful to focus first on the migratory behavior of individuals, as this underlies the collective aspects.

Further, because natural selection acts primarily on individuals, understanding the function of migration, and how migration systems are maintained and evolve, will ultimately concern the genotypes and phenotypes of individual migrants Dingle Animals employ movement for a variety of purposes, but probably most frequently in connection with the use of resources.

These include food, shelter, and mates, all of which are included within an individual's home range, which is in turn located—along with many others—within a habitat that provides the necessary requirements for breeding or maintenance or both.

Movements can be divided into those that occur generally within the home range and those that take the individual more or less permanently beyond it Dingle Foraging is concerned with finding and appropriating resources a food item, say, or a mate and is characteristically meandering and repetitive on short timescales and small spatial scales, the animal changing course frequently as it finds and moves between items.

Dramatic examples include the mass daily vertical movements of plankton and the several-thousand-kilometer foraging round trips extending over several days made by albatrosses Diomedea spp. Ranging implies an exploratory component and takes the organism beyond the current home range to settle eventually in a new one.

We define the behavior as one that ceases once a suitable new home range a resource is found, and a large literature Gandon and Michalakis , Bullock et al. Migration is movement away from the home range that does not cease, at least not initially, when suitable resources or home ranges are encountered.

Eventually, however, the migrant is primed to respond to appropriate resources. This sequence of inhibition and then priming of response assures that the migrant escapes from a region of deteriorating habitats, as would occur for temperate-zone birds in the autumn, for example, and extends its movement to a region where habitats will remain or are becoming favorable Kennedy , Dingle ; see the discussion of preemption, below.

In short-lived insects, the abandonment of home range by migrants is in fact permanent; in long-lived vertebrates, migrants with strong philopatric tendencies i. Thus repeat migrations are often a consequence of long life spans. Ecologically, migration occurs between habitat regions, whereas ranging occurs between habitat units within a habitat region. There will most likely be cases where the distinction between migration and ranging is not obvious, and these may repay careful examination from both behavioral and ecological perspectives.

The qualitative characterization of movement behaviors draws on both form and function, but it seems probable that objective distinctions could be made from observations of the organism's lifetime track, the time series of its successive locations Baker With modern tracking technologies, such an analysis is increasingly practicable, at least for larger vertebrates Kenward et al.

When an animal is nomadic, and home ranges and breeding areas and times are unpredictable, the trajectories of the three types of movement may overlap to some degree. In any case, the distinctions are to be recognized from the patterns of relocations or behaviors and not by their absolute scale: One species's ranging may be as extended temporally, spatially, or both as another's migration. The scale characteristic of migration is more apparent when considered in relative terms.

The spatial extent of an animal's migrations is generally greater than that of any other type of movement it makes, and its periodicity is generally that of one or the other of the two longest timescales most animals experience, the annual cycle and the animal's lifetime.

Animals that complete only a single migration cycle or only part of one include almost all insects, Pacific salmon, and the African black oyster-catcher Haematopus moquini. For example, in the latter species, newly fledged young migrate from their birthplace on the south coast of South Africa to lagoons on the west coast of Namibia, where they remain for two to three years before returning to their natal area to breed without ever migrating again Hockey et al.

Because individuals manifest their capacity for migration through behavior, it is through behavioral traits in particular although not exclusively that natural selection will act to shape migration. Of the various organizational levels, the behavioral is the highest that concerns individuals. Behavior is also observable, although in wild populations where migration is high in the atmosphere, at night, or over or within oceans, the challenge is considerable.

In insects, laboratory studies using wind tunnels Hardie and Powell or flight mills or balances Cooter , Han and Gatehouse are more practicable and allow experimentation as well as observation. It is now possible to study birds in wind tunnels as well Pennycuick et al. On the basis of a pioneering study of the aphid Aphis fabae , in which he monitored locomotory activity and responses to resources in a laboratory flight chamber, J.

As with many insects, A. The sentence about station-keeping responses summarizes Kennedy's experimental findings that at the initiation of migratory flight, and during its early stages, the aphids would ignore an environmental cue a young bean leaf to which they would normally respond by settling and starting to feed, but that after some period of flight they would again become responsive even hyperre-sponsive to this. This is a refined expression of the frequent observation, made for a variety of taxa from butterflies to African ungulates, that migratory movement is characteristically undistracted Dingle Kennedy stressed that because the inhibition of station keeping is temporary, it can be repeatedly switched on and off.

Thus a migrant may switch between migration and foraging while en route, as in the well-known stopovers of migratory birds. As well as responses to cues for initiating and terminating locomotion, the migration syndrome includes endogenous mechanisms for priming and inhibiting these responses, and metabolic and hormonal shifts necessary to prepare the migrant for its journey Dingle These traits are integrated into the organism's life history; for example, it is not only feeding that is inhibited but also maturation and reproduction Kennedy , Ramenofsky and Wingfield It is interesting that migration syndromes have evolved repeatedly and across many taxa.

Phylogenetic studies fail to reveal a deeply embedded ancestral pattern Piersma et al. Underlying the migration syndrome is a genetic complex figure 1 that incorporates both genes and genetic architecture van Noordwijk et al. This too is accessible to experimentation through artificial selection and crossing trials, mostly in insects Han and Gatehouse , Dingle but also in birds such as the black-cap warbler Sylvia atricapilla ; Pulido and Berthold Such experiments have demonstrated correlations between migration-related traits and between migration and reproduction, both indicative of a coadapted suite of traits.

Other expected properties of syndromes, such as trade-offs and the suboptimal adaptations that these imply, are discussed by Sih and colleagues , Pulido , and Roff and Fairbairn Southwood showed that in insects migration is associated with impermanent habitats.

Southwood and Solbreck later made clear how changes in habitat favorability in both time and space now versus later, and here versus elsewhere drive the evolution of migration and dormancy. Migration can be viewed as an adaptation specific to arenas in which changes in habitat quality in different regions occur asynchronously so that movement allows a succession of temporary resources to be exploited as they arise.

It thus involves both escape and colonization. At a minimum, a habitat must enable survival; better-quality habitats will allow development, physiological sequestering of resources, and breeding. Individuals unable to locate a sequence of such habitats will fail to produce offspring.

The members of a migrant population are therefore directly subject to natural selection by the arena through which they travel. Selection will arise from the pattern and timing of the development and decline of favorable habitats, from the incidence of inclement conditions, and, for weak flying and drifting migrants especially, from the incidence of winds and currents in appropriate directions.

Population pathways can be expected to match those features of the arena that affect the migrants in essentially all their properties, including spatial and temporal scale, direction and seasonality, and degree of variability.

At high latitudes, the migratory adaptation will be primarily to the predictable warm—cold seasonal variations; pathways can be expected to show little spatial variance and to be closely synchronized with the annual cycle. Temperate-zone bird migrants are the classical example Newton and Dale If climate changes, timing of migration and resource availability may become mismatched and the population may decline Both et al.

At the other extreme, in semiarid regions, where erratic rainfall is the main determinant of habitat favorability and temperatures are rarely limiting, trajectories may vary greatly; examples include desert locusts Schistocerca gregaria , quelea birds Quelea quelea ; Cheke and Tratalos , and Australian bird migrants Griffioen and Clarke As noted above, one key component of a strategy for surviving in a spatiotemporally varying arena is preemption, in which habitats are abandoned before their quality has declined too seriously.

Preemptive departure may be adaptive for at least three reasons: 1 If an animal waits until quality is poor, it may not be able to accumulate fuel for migration, or habitats at locations within its range may also have deteriorated; 2 if it starts a new breeding attempt, its offspring may be insufficiently developed to depart before conditions become lethal; and 3 early departure may provide it with the advantage of being an initial colonizer of a habitat that is just becoming favorable rather than a late arrival facing established competitors.

Preemption requires precisely the temporary inhibition of station-keeping responses observed by Kennedy in his aphid experiments and incorporated into his definition of migration. In appropriate circumstances, the advantages conferred by preemptive departure will favor migration over extended foraging or ranging as an adaptation.

For much of its history, ecology has focused more on population dynamics over time than on changes in spatial distributions.

In this perspective, changes in population size arise both through births and deaths and through emigration and immigration to and from external sinks and sources Thomas and Kunin By recognizing the spatial dimensions and adding movement to the dynamics, it becomes evident that a cohort of individuals and their descendants form a population regardless of where they are, and follow a trajectory through both space and time Taylor and Taylor , Solbreck , Taylor , Drake et al.

Viewed from this perspective, emigration and immigration fuse, and the movements of individuals maintain the structure and cohesion of populations and determine their connectivity Webster and Marra , Cheke and Tratalos Movement is now seen as a key process in its own right and not just an alternative mechanism contributing to effective birth and death rates.

Depending on the extent of spatial variation and of developmental desynchronization, temporarily distinct subpopulations may form and remain isolated for one or more generations. The population trajectory then consists of a number of strands that may split and merge through space and time, and when mapped, the resulting three-dimensional pathway forms a tangled reticulum that has been analogized to a fern stele Taylor and Taylor Analysis of population trajectories reveals the processes leading to a reticular form.

Classic examples are various species of locust and any of several species of nomadic and seminomadic birds Cheke and Tratalos Population trajectories of such nomads typically differ between years as resources vary, but often an overall to-and-fro trend in the pathway can be discerned Griffioen and Clarke , Deveson et al.

The distributions of aphids and other insects over the British Isles display reticular patterns over seasons and years, and populations are resupplied each spring by infusions from the continent Taylor On a smaller scale, migration allows the milkweed bug Lygaeus equestris to exploit isolated habitat islands of a few square meters each on limestone outcrops in Sweden Solbreck With migration, accompanied by temporary suppression of reproduction, the bugs track resources including single milkweed plants , avoid intraspecific competition, and overwinter in microclimatically suitable diapause sites among the rocks.

The case of L. Metapopulations are the subject of a large literature Hanski , Ovaskainen and Hanski that concerns populations spread over an ensemble of separated habitats in which colonization and extinction of subpopulations occur, with connectivity between subpopulations maintained by movement. The principal application to conservation in fragmented landscapes is easily appreciated. Although it is unlikely that migration in our sense contributes to structure in most metapopulations so far studied but see Cheke and Tratalos , it would be interesting to know how frequently migratory behavior occurs within metapopulations in nature.

In any event, the focus has usually been on the outcomes of movements in the form of colonizations and the contribution of departures to local extinctions, and field data are usually based on mark—recapture methods or surveys rather than direct observation of movement behavior. We agree with Thomas and Kunin that studies of spatiotemporal population dynamics need to examine behavioral processes as well as patterns and outcomes. Both distant and local environments will of course determine year-to-year variation in mortality and breeding condition.

Young that have not bred previously will most likely seek a breeding site by ranging over suitable habitat either before departing or after completing their round-trip annual migration Winkler , but the behavior of settling is not well studied.

The adaptation that steers the migrants along their pathways is presumably coded genetically, so offspring of crosses between members of different subpopulations may be poorly adapted to find either parental breeding site and therefore selected against.

Such processes may help to maintain subspecies or even lead to full speciation Irwin and Irwin Alternatively, extensive mixing may limit the development of complex or differentiated migration pathways, as seems to be the case with monarch butterflies Danaus plexippus ; Brower and Boyce , Shephard et al.

In addition to describing their dynamics in time and space, populations can be characterized by the mean distance among the individuals within them Southwood When animals are attracted to each other, they congregate; if they gather in the same habitat, they aggregate.

In either case, the mean distance among individuals decreases, at least on a local scale. In contrast, behavioral responses can result in animals in a population moving apart, in which case they disperse.

Although they increase the distance between young and their parents, and between the young of a clutch or litter if departures are in different directions, the movements of unrelated young will cross, and young that end up in a particular locality will have arrived from different directions—an example of convergent movement.

Overall, the population experiences little change in the mean distance between individuals; it therefore does not exhibit dispersal in the original sense. The functions of these movements appear to be reoccupation of vacant habitat units, readjustments to local changes in habitat quality, and avoidance of kin competition and inbreeding Gandon and Michalakis At the population level, they result in mixing, the internal redistribution of individuals that leaves the location and spatial extent of the population unchanged.

Population mixing can also occur with migration e. We note first that a definition of a trait or syndrome in biology should provide clear indication that it can respond to natural selection. With few exceptions, that means the definition must be couched in terms of individuals. This is no less true for migration.

Selection has produced specific behaviors and responses to the environment to solve common problems that distinguish migration from other forms of movement Kennedy , Dingle At the same time, it is equally clear that migration is an important population phenomenon. Individual behaviors produce a population outcome, and that outcome provides the selection acting back on individuals.

Although migration can be defined only for individuals behavior, syndromes , it can be described in terms of population outcomes dynamics, trajectories, displacements; table 1 ; Gatehouse A great advantage of the definition of migration from Kennedy cited above is that it invites direct empirical investigation at behavioral and lower organizational levels, potentially leading to several productive lines of research.

There are explicit hypotheses to test using, for example, straightforward at least in principle experiments of a stimulus—response type. Although not all aspects of the syndrome may be present in any one migrant, experiments should still distinguish migratory from other types of movement behavior by revealing the suppression of responses to resources. An obligate migrant will continue moving and bypass breeding sites or food while on its trajectory.

Well-known migrants, such as arctic terns Sterna paradisea and salmon Oncorhynchus and Salmo spp. Like any good hypothesis, Kennedy's definition identifies means by which it can be falsified. Thus it can be modified, improved, or replaced, but to accomplish these, any proposed alternative must distinguish individual migrants from those undertaking other kinds of movement and provide clear means, at least implicitly, by which empirical observation or experiment can make the distinction.

If definitions are too narrow, we need to know what characteristics should be included to broaden them. What is obvious is that analyses of migration as an explicit behavioral syndrome are all too rare. We do not minimize the methodological difficulties in performing such analyses, but neither do we apologize for the complexities presented by migration. Our second observation is that studies of animal movements, including migration, need to be more broadly based. Students of migration need to focus more on migration as a behavioral, ecological, and evolutionary phenomenon rather than as an event that occurs in a particular taxon.

The use of movement to exploit separated and ephemeral habitats and resources transcends particular species and taxonomic groups, and its investigation should do likewise. Much could be learned and heuristic approaches developed from a comparative view. New Word List Word List. Save This Word! We could talk until we're blue in the face about this quiz on words for the color "blue," but we think you should take the quiz and find out if you're a whiz at these colorful terms. See migrate , -ion.

Words nearby migration mignonne , migraine , migrant , migrate , migrating abscess , migration , migration inhibition test , migratory , migratory locust , miguelet , MIG welding. Words related to migration exodus , flight , journey , movement , shift , transfer , diaspora , hegira , move , passage , trek. How to use migration in a sentence If they follow through on that intention, that would be a massive migration , and bad news for cities.

Over a third of executives still have no timeline for reopening their offices Alan Murray September 4, Fortune. First he took energy trading and the NYSE electronic. Are cities really on the edge of a mass exodus? Lance Lambert September 1, Fortune.