Greenery begins in a midsummer in the middle of a winter. One December, in midsummer South Africa, Tim Dee watched swallows and those birds set him off on a journey in pursuit of the spring as it moves north, bringing swallows and all the other spring migrant birds out of Africa and into Europe.

Spring moves north across the Europe from the Mediterranean to the Arctic Ocean at roughly fifty kilometres a day between the winter and the summer solstice. We could call that four kilometres an hour for twelve hours each day. Spring, therefore, moves north at about walking pace.

Greenery follows swallows and other favoured birds out of Africa from their wintering quarters in South Africa, through their staging places in Chad and Ethiopia, across the colossal and incomprehensible Sahara, and on into Europe. It tries to keep company with the birds and with other animals including some people for whom spring has been the determining season. We hear from a Sámi reindeer herder, a swallow-devotee, an Egyptian taxi driver, a chronobiologist in arctic Norway. We read of the spring-seeking D. H. Lawrence and of the poet Samuel Taylor Coleridge. Migrant storks join the swallows and venture the Straits of Gibraltar. Migrant honey buzzards dodge Sicilian hunters and the lava wastes of Mount Etna. A wait in a hide for a bear that does not come allows a vision of how nature goes when we are not there to crowd it out. On the other side of the European continent, the curious North Sea island of Heligoland is a haven for sea-going landbirds on their tricky northbound journeys. There are bears, there are boars, there are reindeer, there are camels, there are elephants, there are ostriches… A diary of the spring's arrival and passage through Britain interleaves the continental greening.

Greenery ends where the greenery of the European spring ends: on the shores of the Arctic Ocean in northern Scandinavia, where, yes, there are swallows in midsummer as there were in Cape Town in December.