They would answer the call of the north in the spring, but in the fall they would come back, barking and whooping and honking in the autumn sky…birds that he well remembered and recognised from the previous year. And this made Rhayader happy, because he knew…the coming of the grey skies and the winds from the north, would send them unerringly back to him.

a. Introduction

Humans have climbed the highest mountain, crossed the largest desert, navigated the widest oceans; but such physical feats are part of the course of life for billions of creatures, who undertake them each year, starting when they are a matter of weeks old.

Every autumn, hundreds of millions of European freshwater eels leave their ponds and river dwellings in northern and Western Europe, and migrate downstream to the sea. Once at sea, they continue swimming 3,000km across the Atlantic to the Sargasso Sea near the West Indies. The adults spawn about 800m below the surface and then die. Their offspring, only an inch long and now in their first summer, will start their migration to Europe - a journey so epic it takes them the first three years of their lives, and they arrive just 7 cm long.

Migration is found in all types of creatures. Single-celled algae make daily migrations to the sea surface to capture the sun's energy before retreating back to the depths. Humpback whales travel up to 8,500km each year to feed on seasonal concentrations of fish and krill. Whales are the largest creatures on our planet, and the subject of much research, yet many of their migration paths remain a mystery. Of these, the largest creatures ever to have lived on Earth, blue whales can grow up to 33m, the length of 9 family sized cars; the location of their breeding grounds, however, remains unknown.

Whilst long-distance, seasonal migration is found in all systematic classes, it is most complex, widespread and spectacular in birds: of the approximate 9,000 known bird species, roughly half show some degree of migratory movements, which translates to about 50 thousand million individuals!