Every autumn, the swallow may fly south with the sun. It is joined by the house martin, the plover, and hundreds of other species of birds. After spending the summer in temperate breeding grounds, where both daylight and food are plentiful, they head south before both resources fade in the winter. When spring returns, so do these migrants.

Their treks can be epic. The bar-tailed godwit flies from Alaska to New Zealand. The Arctic tern makes a 70,000 kilometre round-trip from one pole to another. Even less ambitious migrations still involve small birds, the size of your hand, crossing whole continents.

Migration evolved from stagnation. The ancestors of these birds stayed in the same place all-year round, and gradually, they