Animal migration

Migration is a regular, usually seasonal movement of animals from one area to another and back. It differs from dispersal, when young animals leave home permanently, and from nomadic wandering in search of food.

Migrations happen in every major animal group: birds, mammals, fish, insects, reptiles and even plankton, which migrate up and down in the water each day.

The basic reason is that the best place to feed is not always the best place to breed, and conditions change with the seasons. Many birds breed in the far north in summer, when long days produce a burst of insects and plants and there are fewer predators, then fly south before winter makes food scarce.

Migration is costly and dangerous, so it persists only when the benefits outweigh the risks. Some populations of the same species migrate while others stay put, depending on how harsh their winters are.

• Arctic terns travel between Arctic breeding grounds and Antarctic waters, flying tens of thousands of kilometres a year.
• Bar-tailed godwits have been tracked flying nonstop from Alaska to New Zealand, more than 11,000 kilometres, without feeding.
• Humpback whales swim thousands of kilometres between polar feeding waters and tropical breeding areas.
• Monarch butterflies of eastern North America fly up to several thousand kilometres to overwinter in the forests of central Mexico.
• About 1.5 million wildebeest, with zebras and gazelles, circle through Tanzania's Serengeti and Kenya's Maasai Mara following the rains.

Animals use several navigation systems, often combined. Many birds use the sun as a compass, correcting for its movement across the sky with an internal clock. Night migrants use the stars, learning the point in the sky around which the stars rotate.

Earth's magnetic field provides another cue. Birds, sea turtles and salmon can sense it, and experiments suggest turtles and salmon may recognise the magnetic signature of the places where they were born. Landmarks, coastlines, smells and even low-frequency sounds also help. Salmon use smell to find their exact home stream after years at sea.

Before migrating, many birds eat intensely and can nearly double their body weight in fat, which is the most energy-dense fuel. Some shrink their digestive organs during the flight, then regrow them on arrival.

Monarch butterflies show a different trick: no single butterfly completes the full round trip. The generation that flies south lives much longer than summer generations, and it is their great-grandchildren that return north the following year.

Migratory animals depend on several places, so losing any one of them, a breeding ground, a wintering site or a stopover where they refuel, can cause population declines. Wetlands that shorebirds use as stopovers are especially threatened.

Other dangers include collisions with buildings and power lines, light pollution that confuses night-flying birds, dams that block fish, fences that cut off mammal routes, and climate change, which can shift the timing of food supplies so that migrants arrive too late. Protecting migrants requires cooperation across borders, through agreements such as the Convention on Migratory Species.