Animal Migration

Every autumn, a monarch butterfly that has never left its birthplace in Canada sets off south. It weighs half a gram. It will fly up to 4,800 kilometres to a handful of mountain forests in central Mexico that it has never seen and that no monarch alive has seen, because the butterflies that flew north in spring died generations ago. It will arrive at the same trees. How it does this, and how swallows, salmon, whales, caribou and sea turtles do the equivalent, is one of the most remarkable stories in biology.

Why move at all

Migration is expensive and dangerous. Animals do it because the world is seasonal. Food that is abundant in the Arctic summer vanishes under snow; rain that greens one part of the Serengeti falls somewhere else six months later; the warm shallow seas where whales can safely give birth hold little food. Rather than endure the lean season, migrants follow the good times around the planet, trading the cost of travel for a year of plenty. Many also move to breed somewhere safer than where they feed. Natural selection has honed these journeys over millions of years, so that the timing and the routes are often inherited.

The great journeys

On the plains of East Africa, about 1.5 million wildebeest, joined by zebra and gazelle, march in a thousand-kilometre loop each year, following the rains and the fresh grass they bring, crossing crocodile-filled rivers in their thousands. In the Arctic, caribou herds travel up to 5,000 kilometres between winter forests and summer calving grounds on the tundra, the longest land migration of any mammal.

In the oceans, humpback whales swim from the krill-rich waters of the poles to tropical lagoons to give birth, 8,000 kilometres each way, fasting for months. Grey whales do the same along the Pacific coast of North America. Sea turtles hatched on a beach disappear into the ocean for decades and then return, as adults, to lay their eggs on the very same stretch of sand. Salmon leave the stream where they hatched, spend years in the open ocean, then come back to that exact stream, leaping waterfalls, to spawn and die.

Birds are the champions. The Arctic tern breeds near the North Pole and winters near the South, flying as much as 90,000 kilometres a year. The bar-tailed godwit crosses the entire Pacific in one flight of eleven days. Each spring and autumn, several billion birds cross the Mediterranean and Sahara between Europe and Africa, many of them small songbirds flying alone at night.

The biggest migration of all is one almost nobody sees. Every evening, across every ocean, trillions of small animals, krill, copepods, squid and fish, rise hundreds of metres from the dark depths to feed near the surface, and every dawn they sink again to hide from predators. In sheer weight of life on the move, nothing else comes close.

Finding the way

Migrating animals carry several compasses and at least one map. Many birds and insects use the Sun, correcting for its movement across the sky with an internal clock; monarchs do this with their antennae. Night-flying birds steer by the stars, learning the pattern of rotation around the pole as chicks. Birds, turtles, salmon and even some insects sense the Earth's magnetic field, apparently through specialised molecules in the eye or iron-containing cells, and can read both its direction and its strength, which varies with latitude, giving them a rough idea of where they are.

Smell matters more than anyone expected. Salmon imprint on the chemical signature of their home stream as young fish and follow it upriver years later. Pigeons and seabirds navigate partly by scent. On top of all this, older animals remember landmarks, coastlines and routes, and in species that travel in groups, from whales to geese, the young learn the way from their elders. Where those elders are lost, the knowledge can be lost with them.

Journeys interrupted

Migration routes evolved over millennia and do not easily change. Fences, roads, dams and cities now cut across them. Dams block salmon from their spawning grounds; lights lure sea turtle hatchlings away from the sea; wind farms and glass towers kill birds; the forests where monarchs winter are logged. Climate change is shifting the seasons so that migrants arriving on their ancient timetable find the insects they came to eat have already peaked. Many of the world's great migrations have shrunk, and some have disappeared within living memory.

Why it matters

Migrations knit the planet together, carrying nutrients from oceans to rivers, pollen across continents, and energy from one ecosystem to another. They are also among the oldest and most reliable rhythms of the living world. A swallow arriving in April or a salmon leaping a weir in autumn is proof that a web of seasons, routes and instincts stretching across hemispheres is still, for now, intact.