Each year, billions of animals undertake journeys of extraordinary length, navigating across continents and oceans with a precision that continues to astonish biologists. Animal migration — the seasonal, cyclical movement of individuals or populations from one region to another — is among the most spectacular phenomena in the natural world, and understanding its mechanisms has occupied scientists for centuries. Though often associated with birds, migration occurs across an enormous range of taxa, including insects, fish, marine mammals, sea turtles, and even some species of crabs.

The primary driver of migration is resource availability. Seasonal changes in temperature and daylight alter the abundance of food, water, and suitable habitat across different parts of the world. Rather than endure local conditions when resources become scarce, migrating animals exploit the predictability of seasonal cycles, traveling to regions where resources are temporarily abundant. Arctic terns, for example, breed in Arctic regions during the northern summer, then travel to Antarctic waters — a round trip of approximately seventy thousand kilometers — to take advantage of the southern summer feeding grounds. This journey makes the Arctic tern the animal with the longest known migration route.

Navigating across such vast distances requires sophisticated orientation mechanisms. Research has revealed that migrating animals use multiple complementary cues: the position of the sun and stars, the detection of Earth's magnetic field, olfactory cues, and visual landmarks. Many species appear to possess a magnetic sense — an ability to detect the intensity and inclination of Earth's magnetic field, providing them with both a compass direction and, in some cases, map information about their geographic position. Studies of homing pigeons and European robins have shown that disrupting magnetic cues using artificial magnetic fields causes disorientation, while birds exposed to shifted magnetic fields adjust their orientation accordingly.

Insect migration demonstrates that impressive navigational feats are not limited to vertebrates. The monarch butterfly undertakes a migration of up to four thousand kilometers from its summer range in the northern United States and Canada to overwintering sites in the oyamel fir forests of central Mexico. Remarkably, the butterflies that make the southward journey in autumn are several generations removed from the individuals that completed the previous northward migration — no single butterfly makes the round trip. The navigational information must therefore be at least partly encoded genetically rather than learned from experience. Scientists have found evidence that monarchs use a time-compensated sun compass, adjusting their orientation based on the sun's position relative to the time of day.

Fish migrations, particularly those of anadromous species — fish that are born in fresh water, spend their adult lives in the ocean, and return to fresh water to spawn — have long fascinated biologists. Pacific salmon navigate from their ocean feeding grounds to the precise freshwater streams where they were born, sometimes traveling hundreds of kilometers upstream. Research has demonstrated that salmon use olfactory imprinting — they memorize the chemical signature of their natal stream as juveniles and use that olfactory memory to locate it years later as adults. The return journey is powered by physiological changes that prepare the salmon's body for the extreme demands of upstream travel and, ultimately, reproduction and death.

Migration imposes significant costs on individuals. The energy demands of long-distance travel are enormous, and migrants must accumulate substantial fat reserves before departure. Many birds double their body mass in preparation for nonstop transoceanic flights. Mortality during migration is high, particularly for inexperienced juveniles making their first journey. Additionally, migration exposes animals to unfamiliar habitats and novel predators. Yet despite these costs, migration has clearly been favored by natural selection in many lineages, indicating that the fitness benefits — access to superior breeding grounds, reduced parasite loads in seasonally cold environments, and rich temporary food sources — outweigh the substantial risks of the journey.

Climate change is increasingly disrupting long-established migration patterns. As temperatures shift, the timing of peak resource availability is changing faster in some regions than in others, creating mismatches between the arrival of migrants and the abundance of the food sources they depend upon. Some species have shown flexibility, adjusting their migration timing in response to warming conditions; others appear unable to adapt quickly enough, leading to population declines. The integrity of migration corridors — the chains of stopover habitats that migrants depend on for rest and refueling — is also threatened by habitat loss and fragmentation along migration routes.