Animal migration

Migration is a general term employed by ecologists and ethologists to describe the nearly simultaneous movement of many individuals or entire populations of animals to or between different habitats. As defined, migrations do not include local excursions made by individuals or small groups of animals in search of food, to mark territorial boundaries, or to explore surrounding environments.

Nomads are migrants whose populations follow those of their primary food sources. Such animals (the American bison, for example) do not have fixed home ranges and wander in search of suitable forage. Some scientists view nomadic movements as a form of extended foraging behavior rather than as a special case of migration. In either context, the important point is that populations change habitats in response to changing conditions.

In contrast to migrations made by populations and excursions made by individuals, the spreading or movement of animals away from others is known as dispersal. Examples of dispersal include the drift of plankton in currents and the departure of subadult animals from the home range of their parents. In numerous species (sea turtles, rattlesnakes, and salmon, for example), dispersed members of a population may return to the place of origin after a variable interval of time.

Means and Reasons

Some migratory species can orient themselves—that is, they know where they are in time and space. Many birds and mammals, for example, have an inherent sense of the direction, distance, and location of distant habitats. Orientation and travel along unfamiliar routes from one place or habitat to another is called navigation. Navigators use environmental and sensory information to reach distant geographical locations, and many of them do so with a remarkably accurate sense of timing. Homing pigeons are perhaps the best-studied animal navigators. These birds are able not only to discover where they are when released but also to return to their home loft from distant geographical locations.

Much has been learned about how animals successfully navigate over long distances from the pioneering studies of Archie Carr. Carr proposed that green sea turtles successfully find their widely separated nesting and feeding beaches by means of an inherent clock sense, map sense, and compass sense. His investigations and those of many others continue to stimulate great interest in the physiology and ecology of navigating species and in the environmental cues to which they respond. Sensory biologists, biophysicists, and engineers have incorporated knowledge of how animals detect and use environmental information to develop new and more accurate navigational systems for human use.

Animals use a variety of cues to locate their positions and appropriate travel paths. Most species have been found to use more than one type of information (sequentially, alternatively, or simultaneously) to navigate. Among the animals known to navigate are birds (the best-studied group), lobsters, bees, tortoises, bats, marine and terrestrial mammals, fish, brittle starfishes, newts, toads, and insects. Included among the orientation guideposts that one or more of these groups may use are the positions of the sun and stars, magnetic fields, ultraviolet light, tidal fluctuations caused by the changing positions of the moon and sun, atmospheric pressure variations, infrasounds (very low-frequency sounds), polarized light (on overcast days), environmental odors, shoreline configurations, water currents, and visual landmarks. Celestial cues also require a time sense, or an internal clock, to compensate for movements of the animal relative to changing positions of celestial objects in the sky. In addition to an absolute dependence on environmental cues, young or inexperienced members of some species may learn navigational routes from experienced individuals, such as their parents, or other experienced individuals in the population. Visual mapping remembered from exploratory excursions may also play a role in enhancing the navigational abilities of some birds, fish, mammals, and other animals.

The different categories of animal movements, however, are perhaps not so important as the reasons animals migrate and the important biological consequences of the phenomenon. As a general principle, migrations are adaptive behavioral responses to changes in ecological conditions. Populations benefit in some way by regularly or episodically moving from one habitat to another.

An example of the adaptive value of migratory behavior is illustrated by movement of a population from a habitat where food, water, space, nesting materials, or other resources have become scarce (often a seasonal phenomenon) to an area where resources are more abundant. Relocation to a new habitat (or to the same type of habitat in a different geographical area) may reduce intraspecific or interspecific competition, may reduce death rates, and may increase overall fitness in the population. These benefits may result in an increase in reproduction in the population. Reproductive success, then, is the significant benefit and the only biological criterion used to evaluate population fitness.

Programmed and Episodic Movements

While many factors are believed to initiate migratory events, most fall into one of two general categories. The first and largest category may be called programmed movements. Such migrations usually occur at predictable intervals and are important characteristics of a species’ lifestyle or life cycle. Programmed migrations are not, in general, density-dependent. Movements are not caused by overcrowding or other stresses resulting from an excessive number of individuals in the population.

The lifestyle of a majority of drifting animals whose entire lives are spent in the water column, for example, includes a vertical migration from deep water during the day to surface waters at night. Thus, plankton exhibit a circadian rhythm (activity occurring during twenty-four-hour intervals) in their movements. An abundance of food at or near the surface, and escape from deep-water predators, are among the possible reasons for these migrations. Daily vertical movements of plankton are probably initiated by changes in light intensity at depth, and the animals follow light levels as they move toward the surface with the sinking sun. It is interesting to note that zooplankters living in polar waters during the winter-long night do not migrate.

Monarch butterflies and many large vertebrates, such as herring, albatross, wildebeests, and temperate-latitude bats, migrate from one foraging area to another, or from breeding to foraging habitats, on a seasonal or annual basis. Annual migrations usually coincide with seasonal variation. Changes in day length, temperature, or the abundance of preferred food items associated with seasonal change may stimulate mass movements directly, or indirectly, through hormonal or other physiological changes that are correlated with seasonal environmental change. The onset of migration in many vertebrates is evidenced by an increase in restlessness that seems, in human terms, to be anticipatory.

In addition to their daily vertical migrations (lifestyle movements), the life cycles of marine zooplankton involve migrations, and it is convenient to use them as examples. As discussed, most adult animal plankters are found at depth during the day and near the surface at night. In contrast, zooplankton eggs and larvae remain in surface waters both day and night. As the young stages grow, molt, and change their shapes and food sources, they begin to migrate vertically. The extent of vertical migrations gradually increases throughout the developmental period, and as adults, these animals assume the migratory patterns of their parents. Patterns of movement that change during growth and development are examples of ontogenetic, or life cycle, migrations.

The second large category of migratory behavior includes episodic, density-dependent population movement. Such migrations are often associated with, or caused by, adverse environmental changes (effect) that may be caused by overlarge populations (cause). Local resources are adequate to support a limited number of individuals (called the “carrying capacity” of the environment), but once that number has been exceeded, the population must either move or perish. Unfortunately, migration to escape unfavorable conditions may be unsuccessful, as another suitable habitat may not be encountered. Migrations caused by overpopulation or environmental degradation are common. Pollution and habitat destruction by humankind’s activities are increasingly the cause of degraded environments, and in such cases, it is reasonable to conclude that humans have reduced the carrying capacity of many animal habitats. Familiar examples of density-dependent migrations are those of lemmings, locusts, and humans.

Further research indicates that migrations also result from acquired or learned behaviors. In a study of white stork migrations in Europe from 2013 to 2020, scientists used tracking and monitoring technology to collect data concerning the birds’ migration pathway, time in flight, pace, and energy expenditure. The results indicated that young birds explore new places and move more slowly, while mature birds use more energy to move quickly on a direct route. As the young birds age, they implement shortcuts into their migration route that were not previously used by other birds, potentially indicating the use of knowledge in spatial memories acquired as young birds. These researchers note that migratory instincts are the foundation of the behavior, but learning new, better routes indicates the presence of a more dynamic migratory process.

Studying Migration

Methods used by scientists to study the mass movements of animals are quite varied and depend on the investigator’s research interests and on the kinds of organisms being investigated. Environmental or physiological factors that initiate migrations may be of interest to sensory biologists and physiological ecologists; knowledge of variation in population distributions is important to biogeographers and wildlife biologists; and migrations in predator-prey relationships, competition, pollution, and life-history strategies are important aspects of classical ecological studies.

In addition to the specific aspect of migration being studied, the particular group of animals under investigation (moths, eels, elephants, snails) requires that different methods be used. Some of the approaches used in migration-related research illustrate how information and answers are obtained by scientists.

Arctic terns migrate from their breeding grounds in the Arctic to the Antarctic pack ice each year. The knowledge that these birds make a twenty-thousand-mile annual round-trip comes from the simplest and most practical method: direct observation of the birds (or their absence) at either end of the trip. Direct observation by ornithologists of the birds in flight can establish what route they take and whether they pause to rest or feed en route. Many birds have also been tracked using radar or by observations of their silhouettes passing in front of the moon at night. Birds are often banded (a loose ring containing coded information is placed on one leg) to determine the frequency of migration and how many round-trips an average individual makes during its lifetime. From this information, estimates of longevity, survivorship rates, and nesting or feeding site preferences can be made.

Factors that initiate migratory behavior in terns and in other birds can often be determined by ecologists able to relate environmental conditions (changes in temperature, day length, and the like) to the timing of migrations. Physiological ecologists study hormonal or other physiological changes that co-occur with environmental changes. Elevated testosterone levels, for example, may signal the onset of migratory behavior.

How Arctic terns orient and navigate along their migratory routes is usually studied by means of laboratory-conducted behavioral experiments. Birds are exposed to various combinations of stimuli (magnetic fields, planetarium-like celestial fields, light levels), and their orientation, activity levels, and physiological states are measured. Experiments involving surgical or chemical manipulation of known sensory systems are sometimes conducted to compare behavioral reactions to experimental stimuli. In such experiments, the birds (or other test animals) are rarely harmed.

Tags of several types are used to study migrations in a wide variety of animals, including birds, bees, starfishes, reptiles, mammals, fish, snails, and many others. Tags may be transmitting collars (located by direction-finding radio receivers); plastic or metal devices attached to ears, fins, or flippers; or even numbers, painted on the hard exoskeleton of bees and other insects. Additional types of tagging (or identifying) include radioactive implants and microchips that can be read by computerized digitizers; the use of brands and tattoos; and, of great interest, the use of biological tags. Parasites known to occur in only one population of migrants (nematode parasites of herring, for example) provide an interesting illustration of how the distribution of one species can be used to provide information about another.

The Importance of Migration

The causes, frequency, and extent of animal migration are so diverse that several definitions for the phenomenon have been proposed. None of these have been accepted by all scientists who study animal movements, however, and it is sometimes difficult to interpret what is meant when the term “migration” is used. Most researchers have adopted a broad compromise to include all but trivial population movements that involve some degree of habitat change.

It is important to recognize that few populations of animals are static; even sessile animals (such as oysters and barnacles) undergo developmental habitat changes, which are referred to as ontogenetic migrations. Aside from certain tropical and evergreen forest areas where migrations are relatively uncommon, a significant number of both aquatic and terrestrial species move from one habitat to another at some time during their lives. In the face of environmental change, including natural events, such as seasonal variation and changes caused by resource limitations and environmental degradation, animals must either move, perish, or escape by means of drastic population reduction or by becoming inactive until conditions become more favorable (hibernation, arrested development and dormancy, and diapause in insects are examples of behavioral-ecological inactivity). Migration is the most common behavioral reaction to unfavorable environmental change exhibited by animals.

One cannot understand the biology of migrators until their distribution and habitats throughout life are known. The patterns of animal movements are fascinating, and it is useful to summarize some of the major differences between them. First, many species travel repeatedly during their lives between two habitats, on a daily basis (as plankton and chimney swifts do) or on an annual basis (as frogs and elks do). Second, some species migrate from one habitat (usually suitable for young stages) to another (usually the adult habitat) only once during their lives (for example, salmon, eels, damselflies, and most zooplankton, which live on the bottom as adults). Third, some species (many butterflies, for example) are born and mature in one geographical area (England, for example), migrate as adults to a distant geographical area (Spain, for example), and produce offspring that mature in the second area. These migrations take place between generations. In a fourth pattern, one may include the seasonal swarming of social arthropods, such as termites, fire ants, and bees. A fifth but ill-defined pattern is discernible, exemplified by locust “plagues,” irruptive emigration in lemmings and certain other rodents, and some mass migrations by humankind, as caused by war, famine, fear, politics, or disease. These are episodic and often, if not primarily, caused by severe population stress or catastrophic environmental change.

Principal Terms

Biological Clock: An inherent sense of timing regulating certain types of behavioral activity, such as migration

Clock Sense: An inherent awareness of time or time intervals used, for example, to compensate for celestial movements in navigation

Dispersal: The spreading apart of individuals away from one another and away from a place; includes a directional component when passive animals are moved by winds or currents

Habitat: A specific, recognizable geographical region in which a particular kind of organism lives

Migrant: A group or species of animal that moves from one habitat or geographical region to another

Navigation: To follow or control the course of movement from the place of origin to a specific destination

Nomads: Migrants without a specific habitat; wanderers

Orientation: An inherent sense of geographical location or place in time

Population: A group of individuals of the same species geographically located in a given habitat at the same time

Bibliography

Able, Kenneth P., ed. Gatherings of Angels: Migrating Birds and Their Ecology. Frisco, Comstock, 2003.

Aidley, David, ed. Animal Migration. Cambridge, Cambridge University Press, 1981.

Begon, Michael, John Harper, and Colin Townsend. Ecology: Individuals, Populations, and Communities. 5th ed. Oxford, Sinauer Associates, 2021.

Bradley, Doug. Animal Migration. Chicago, Britannica Educational Publishing, 2024.

Dingle, Hugh. Migration: The Biology of Life on the Move. 2nd ed. Oxford, Oxford University Press, 2014.

Eisner, Thomas, and Edward Wilson, eds. Animal Behavior: Readings from “Scientific American.” New York City, W. H. Freeman, 1975.

Nelson, Bryan. "14 of the Greatest Animal Migrations." Tree Hugger, 30 Aug. 2024, www.treehugger.com/greatest-animal-migrations-4869293. Accessed 10 Sept. 2024.

Nordell, Shawn E., and Thomas J. Valone. Animal Behavior: Concepts, Methods, and Applications. 4th ed., Oxford, Oxford University Press, 2024.

Pyle, Robert Michael. Chasing Monarchs: Migrating with Butterflies of Passage. New Haven, Yale University Press, 2014.

Rankin, Mary, ed. Migration: Mechanisms and Adaptive Significance. Marine Science Institute, Austin, University of Texas at Austin, 1985.

Reader’s Digest Association. The Wildlife Year. 1993.

Schone, Hermann. Spatial Orientation: The Spatial Control of Behavior in Animals and Man. Translated by Camilla Strausfeld. Princeton, Princeton University Press, 2014.