We are steering through the lodes, through the tall rushes where the reed warblers nest, looking for the first cuckoo of spring. Above us drift broad branches, clouds of midges and the pale Cambridgeshire sky.
This is, traditionally, the week of the first cuckoo in Britain. It arrives on our shores having flown more than 6,000 miles from Africa, and Wicken Fen wetlands, a National Trust nature reserve just outside Ely, is its first port of call.
On these 1,800 acres live 7,800 species. There are highland cattle and konik ponies, introduced to graze the land. There are deer and grass snakes, caterpillars that live inside the willow trees, false scorpions and great crested newts, otters, water voles, dragonflies, kingfishers, snipe. Stand quite still and you will hear chiff-chaffs, chaffinches, Cetti's warblers; dusk brings marsh harriers, barn owls, bitterns.
The swallows returned early to the fen this year, on 19 March. The celandines are out, the birch and the hawthorn are coming into leaf, and soon there will be marsh orchids in the sedge, wigeons and lapwings nesting. But it is the cuckoo that heralds the true beginning of spring.
The cuckoo is a brood parasite, laying its eggs in another bird's nest, usually the meadow pipit, dunnock or reed warbler. High in the trees above it will wait; when the host bird leaves, down the cuckoo will swoop, with just 10 short seconds to lay its egg in the host nest. By last May there were four reed warbler nests here, and each one held a cuckoo's egg.
The cuckoo's distinctive call has been part of this landscape for centuries. The females make a bubbling sound, while the male's call is the well-known cuc-cooo, ringing out in the key of C major, a descending minor third, G to E. It's a song that has long inspired writers and musicians; Wordsworth called it "the wandering voice". The Yorkshire-born composer Frederick Delius turned it into a tone poem, On Hearing the First Cuckoo in Spring, using oboe, strings, clarinet to replicate the call. As far back as the 13th century there was an English round named Cuckoo Song, a celebration of blooming meadows, farting bucks and greening woods: "Sumer is icumen in," it declares. "Lhude sing Cuccu!"
But cuckoo figures are dwindling in Britain. Each year fewer and fewer birds make the annual migration to these shores. Since the 1960s, numbers have fallen by 59%, and the decline is accelerating - the population is down 37% in the last 15 years.
Today there are somewhere between 10,000 and 20,000 breeding pairs in the British Isles, owing to depleted food and water sources in Africa, global warming, which has led the moth caterpillar - a major cuckoo food source - to hatch too early, and pesticides that have killed off much of the UK's insect population. If the trend continues, the cuckoo will soon be placed on the RSPB's list of most endangered species.
The loss of the cuckoo, along with the rapid decline in the populations of all our native songbirds, the nightingale, the skylark, the turtle dove among them, will of course have profound implications for the British countryside and for future generations - not least, it will untether us from our seasons, unmoor us from our natural cycle. And what it means too is that the way this country sounds, the very music of our land, is changing; there will be no more "lhude" singing in the greening woods.
The reeds need to grow a little taller, a little thicker yet; they're waiting for the colts, the new shoots that rise up higher and keep the reed warbler nests hidden. We keep our eyes trained on the sky, waiting for a glimpse of something grey and dove-sized, with a white barred underbelly; we tilt our heads, raise our ears and listen, still hoping, through the calls of the woodpigeons and the goldfinches and the frogs, to hear the low, sweet call of the first cuckoo.
