Choosing the route
Migrating birds do not just point themselves in the right direction and hope for the best. Each species has its own traditional route.
How migrating birds get from A to B
Most routes follow obvious landmarks such as river valleys or coastlines. Some birds take winding routes around the coast. Others travel more directly, even if this means crossing perilous stretches of desert or sea. Routes often converge at certain junctions, such as mountain passes or narrow sea crossings.
Many migrating birds return along roughly the same route they followed on outward journey. Others return a different way, so their annual journey is roughly circular. This is called loop migration.
Sand martins fly to Africa over the western Mediterranean, passing to the west of the Alps, but return in a loop via the eastern Mediterranean, passing to the east of the Alps. Loop migration allows birds to make use of food supplies or weather patterns that are found in different places at different times.
Around the world
It's not only European birds that migrate; the whole world is criss-crossed with migration routes. In Asia, many northern species spend winter in the tropical south-east.
Some, such as the spine-tailed swift, even get as far as Australia by flying down through the islands of Indonesia. In America, birds from the northern USA and Canada migrate to South and Central America.
America has no seas or deserts to separate the north from the south, so migration is easier for birds that need to feed along the way. This explains why some tropical species from South America, such as the ruby-throated hummingbird, have extended their breeding range as far north as Canada.
Not all migrants head south for winter. Some birds that breed in the southern hemisphere migrate north – such as the carmine bee-eater, which leaves South Africa in March (autumn in South Africa) and heads for east Africa.
The southern hemisphere is like a mirror image of the northern hemisphere: it gets colder as you head further away from the equator. Really these birds are doing the same thing as European breeding birds - finding a winter home with more food and warmth.
Sea or land?
Over the sea
The Mediterranean sea forms a massive barrier for birds migrating between Europe and Africa. Some small birds, with enough energy to keep flapping, cross wherever they can. But many larger birds head for the narrowest crossing points.
Some go west, via Gibraltar, where the coast of Europe is only 25 km (15 miles) from Africa. Others go east, reaching Africa through Turkey and Israel. In spring and autumn, thousands of storks, kites and other large birds gather at these points. They wait for thermals – which only form over land – to lift them up high enough. Then the wind carries them over the sea.
Over the desert
The Sahara desert in North Africa is a vast wilderness of sand, rock and gravel, about as big as the United States. Days are scorching hot, nights are freezing cold, and with very little food or water, it is no place for migrating birds. Unfortunately, the Sahara lies right between Europe and tropical Africa, and more than 500 million birds have to cross it twice a year.
Wading birds such as dunlins avoid the Sahara altogether by flying down the coast, feeding at estuaries along the way.
But many land birds, such as cuckoos, cross it in one non-stop flight. Once they reach the other side, the exhausted migrants drop down in the first green space they find. They feed up quickly to regain energy for the final leg of the journey.