The African-Eurasian bird migration system is one of the earth's great wonders, a gigantic signal of spring and autumn in the northern hemisphere.
It features as many as five billion mainly tiny creatures, which twice a year make epic journeys between their wintering grounds in Africa and breeding grounds in Britain and the rest of Europe.
Imagine: you're the size of a Mars bar, with wings each the size of a credit card, and you set off from Senegal one day to fly to Surrey. That means crossing the Sahara, the world's biggest desert – hope you can find enough food – crossing north Africa, crossing the Mediterranean on those tiny wings, maybe crossing Spain, crossing France, and then crossing the Channel. And eventually you get there, to a wood near Guildford, say, having survived the weather, and exhaustion, and hunters, and predators such as falcons on your 2,000-mile journey – and five months later, having produced your chicks, you have to fly back again.
Tough? Well, that's what life is like if you're a willow warbler, or any of the 48 of Britain's 215 or so breeding bird species which are long-distance migrants that breed here but winter in sub-Saharan Africa. The list includes many of the birds which are most culturally resonant for us: the nightingale, the cuckoo, the swallow, the swift, the majority of our warblers.
In fact they are not really British birds wintering in Africa. It is much more accurate to describe them as African birds summering in Britain.
They pour out of Africa every spring and fly all over Europe to nest, with more than 120 species heading to the continent west of the Urals, and about 200 heading to Eurasia as a whole. They fly north to escape the competition back home, and to profit from the much greater summer day-length of the high latitudes, which offers more time for finding food for hungry chicks. The advantages of coming clearly outweigh the problems of the journey, or they would not do it, and for millions of years they have done it successfully: despite mass mortalities en route, untold numbers of birds get through, and reproduce.
But now the balance may be tipping against them. It may be climate change; it may be habitat loss. No one yet knows, but something is disrupting the whole system, and their numbers are plunging. So if you're in the countryside this week, with many of the birds just arriving back, enjoy the song of the willow warblers, and the cuckoos, and the nightingales if you're lucky. You may have less time to enjoy them than you think.