A food chain is a sequence that demonstrates how one organism forms the food for another. It begins with the simplest animals and plants, and continues until the top of the chain is reached, Humans or predator animals are often at the top of food chains.
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Like migratory birds, there are many animals which return to their birthplace to breed. Even frogs and toads migrate back to the ponds where they first grew up.
Salmon carry out long migrations to return to their birthplace. Young salmon, which hatch in small streams, travel down rivers to the sea as they develop. They spend nearly all of their adult life at sea. When it is time to breed, the salmon migrate back towards their birthplace, sometimes travelling for thousands of kilometres. They somehow manage to enter exactly same river and travel upstream to find the same tiny stream in which they hatched. Here they lay their eggs to repeat the cycle. The adult salmon die after breeding.
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The Arctic tern makes the longest-known migration of any bird we know, by travelling from the Arctic to the Antarctic and back again.
One tern was ringed in the north of England and arrived in Australia 115 days later, having flown an average distance of 160 km per day. The short-tailed shearwater, which breeds on offshore islands near Australia, has an annual migration all round the Pacific. It passes through Japan, Alaska, Canada and Fiji before returning home again to breed.
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For thousands of years, locust swarms have devastated farmland throughout Asia and Africa. A typical swarm can be 50 km long and contain more than 100,000 million locusts. These animals are large grasshoppers that normally live a solitary and harmless existence. When their population builds up to a high level, they begin to mass together and migrate in search of food. These migrations can cover many thousands of kilometres.
Locust swarms can now be predicted by monitoring the insects’ behaviour. Locusts can be sprayed with pesticides to prevent them from swarming.
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Many insects migrate, sometimes for thousands of kilometres. The monarch butterfly migrates from North America to Mexico in the winter, a distance of 3,200 km. Here they spend the winter in a state of semi-hibernation. Incredibly, most of the butterflies die during the journey, and the grandchildren of the original butterflies arrive back exactly where their grandparents hatched. Sometimes it takes three or four generations before the migration is completed. In spring, the butterflies begin the journey north again.
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Every autumn, swallows gather in large flocks to rest before they begin their long migration to Africa. Swallows and their relatives, swifts and martins, all migrate to Africa when the weather becomes too cold for them to catch their insect prey. They return in the spring when the weather in northern Europe begins to warm up.
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