Migration Science: The Biology of Earth's Greatest Animal Journeys

Animal migration — the seasonal movement of animals between different habitats to exploit seasonal resources or avoid unfavourable conditions — is one of the most spectacular phenomena in the biological world. The Great Serengeti-Mara Migration involves 1.5 million wildebeest, 200,000 zebras, and 500,000 gazelles moving in a continuous 800-kilometre circuit following the rains across the Serengeti plains of Tanzania and the Masai Mara of Kenya. The Arctic tern (Sterna paradisaea) makes a round trip of approximately 71,000 kilometres each year between Arctic breeding grounds and Antarctic wintering areas — the longest migration of any animal. Monarch butterflies navigate 4,000 kilometres from Canada to specific overwintering sites in Mexican mountains that no individual butterfly has ever visited before. Each of these migrations requires biological capabilities — navigation, physiology, energy management — that represent some of the most complex feats in the animal kingdom.

Navigation — How Do They Know Where to Go?

The navigational capabilities of migratory animals have long fascinated — and puzzled — biologists. Animals use multiple navigation systems, often in combination: the position of the sun (with internal chronometers to compensate for its movement across the sky); the pattern of stars (used by night-migrating birds to determine geographic north); the Earth's magnetic field (sensed by magnetite crystals in the inner ear of birds and fish, or by light-sensitive cryptochromes in bird retinas); olfactory cues (salmon use the chemical signature of their natal stream to navigate home); and spatial memory of landmarks. Most migratory animals use more than one navigational system simultaneously, cross-checking information from multiple sources and switching to alternative systems when primary navigation fails.

The Energy Economy of Migration

Long-distance migration places extreme physiological demands on animals. A bird that migrates thousands of kilometres must carry enough energy reserves to power the journey — typically stored as fat — while keeping its total body weight low enough to sustain flight. The energy management strategies of migratory birds represent some of the most sophisticated physiological adaptations in vertebrate biology. Before migration, birds enter a state of hyperphagia — accelerated feeding that may increase body mass by 50-100% in fat storage. During migration, they may shrink their digestive organs and reduce other energy-expensive tissues to save weight, then regenerate them at stopover sites. Some species, like the bar-tailed godwit, carry this to an extreme: before their non-stop trans-Pacific flight, they shrink their stomach, intestines, and liver to minimum size, maximising the proportion of body mass available for flight fuel.

The Energetics of Long-Distance Migration

Long-distance migration is among the most energetically demanding activities in the animal kingdom. Bar-tailed godwits flying non-stop from Alaska to New Zealand — a journey of approximately 11,000 kilometres over 8-9 days — must nearly double their body mass before departure by accumulating fat reserves, reduce the size of their digestive organs (which are metabolically expensive to maintain) to free up mass for additional fuel, and sustain continuous flight at an average speed of 55 kilometres per hour for over 200 hours without eating, drinking, or sleeping in any conventional sense. This represents one of the most extreme physiological performances of any vertebrate animal.

The fuelling strategy for long-distance migration is tightly linked to the distribution of stopover sites — locations where migrants can rest and refuel before continuing their journey. The degradation or loss of key stopover sites can have catastrophic consequences for migratory species: the Yellow Sea mudflats of China and South Korea, which are being rapidly reclaimed for development, serve as the critical refuelling stop for over 2 million migratory shorebirds annually. The loss of these mudflats — which support the invertebrate communities on which shorebirds depend — is driving population declines across dozens of species that winter in Australia and New Zealand but breed in Siberia and Alaska.

Navigation Mechanisms — How Migrants Find Their Way

The navigational mechanisms used by migratory animals involve multiple redundant systems that are integrated into a comprehensive spatial awareness. The magnetic sense — detected by magnetite crystals in the beaks of birds and by cryptochrome proteins in the eyes that may allow birds to literally see magnetic field lines — provides a global positioning reference. The sun compass — calibrated against the rotation of the night sky around the celestial pole — provides directional information during daytime flight. The star compass — learned during the sensitive period of early development — guides nocturnal migrants by the pattern of stellar rotation. These systems are supplemented by olfactory cues (used by salmon and some seabirds), infrasound navigation, and the learned memory of familiar landmarks along established routes.

The Mechanics of Long-Distance Navigation

Understanding how migratory animals navigate across thousands of kilometres without GPS or maps has been one of the most productive research areas in animal biology over the past half-century. Multiple navigation systems have been identified, and most species use them in combination — a redundancy that ensures orientation even when one system is disrupted. The sun compass — the ability to use the sun's position in the sky, corrected for time of day using an internal circadian clock — is used by many diurnal migrants, including songbirds, monarch butterflies, and sea turtles. The star compass — orientation using the pattern of stars rotating around the celestial pole — is used by many nocturnal migrants, particularly warblers and thrushes, which imprint on the star pattern during a critical period as juveniles before their first migration. The magnetic compass — orientation using the Earth's magnetic field, detected through magnetite crystals or chemical mechanisms in the eye — is used across an extraordinarily diverse range of taxa, from bacteria and fish to birds, sea turtles, and some mammals.

Perhaps the most sophisticated navigational tool is the magnetic map — the ability to use spatial variation in the Earth's magnetic field not merely as a compass (which direction am I facing?) but as a positional system (where am I?). Research on loggerhead sea turtles has demonstrated that they can determine their approximate latitude and longitude from the combination of magnetic field intensity and inclination angle, and use this positional information to navigate to specific geographic targets. Similar magnetic map abilities have been demonstrated or inferred in sockeye salmon, European robins, and homing pigeons. The molecular mechanisms of magnetic sensing — particularly the radical pair mechanism in cryptochrome proteins in the retina of birds — represent one of the most exciting frontiers in sensory biology.