You think you know the world, at least the general shape of it, the way it works, yet sometimes you are struck by just how far you are from truly comprehending it in all its glorious peculiarity.
For example, have you ever remotely considered that the ocean, the seemingly empty vastness of the waves, might be "overlain with odour landscapes"? What that means is that drifting above the surface of the water are smells, immense blocks or plumes of smells rising and falling like islands, appearing and disappearing like mists, so that the great marine vastness, which seems so featureless, is not really featureless at all – if you can sniff your way around it.
It's an astounding idea, it seems to me, that smells form a landscape, or rather a seascape, superimposed upon the surface of the sea, which can be read and understood. A whole new way of interpreting the world. Wish I had the nose for it. But to make use of the olfactory seascape, to give it its formal name, you have to have the specialised nose of a pelagic (or deep-ocean) seabird like a petrel or an albatross.
Petrels and albatrosses, whose noses are immensely sensitive, can pick up faint odours such as that of dimethyl sulphide, a chemical given off by plankton; and following them upwind, they can find food supplies, or they can even find their way home, sometimes over thousands of miles of empty water. Smell is a key feature of their lives. Yet 50 years ago, most scientists thought that a sense of smell was absent or minimal in nearly all birds.
That we now know very differently is one of the many revelations in a fascinating new book about how birds experience the world. Tim Birkhead is one of our leading academic ornithologists, and his Bird Sense (Bloomsbury, £16.99) is an inspired bringing together of all the latest scientific research on avian sight, sound, touch and taste as well as smell, along with some senses which are beyond human capabilities altogether.
For instance, it is now clear that another way that birds navigate over huge expanses of open ocean is by sensing the earth's magnetic field; and they also orientate themselves by using the sun in the day, and the stars at night. Some birds, such as the oilbirds of South America or the cave swiftlets of South-east Asia, are even capable of flying around confined spaces in pitch blackness by using echolocation – giving off sounds and picking up their echoes, just as bats do.
But even the more familiar senses can be developed in birds, Professor Birkhead points out, to levels far beyond what we are capable of ourselves. Shrikes, which are small predators, can see larger predators such as falcons miles away, before they are at all visible to the human eye.
The great grey owl of the Far North can detect scurrying mice and voles under several inches of snow by using its asymmetric ears – one sited at two o'clock on its head, the other at seven o'clock – which pinpoint precisely the direction the sound is coming from. And the kiwi of New Zealand has grown so dependent on smell and its sense of touch during its nocturnal foraging for earthworms, that its eyes have withered away to almost nothing, and sight is no use to it.
Professor Birkhead leavens his account of advances in sensory biology with an absorbing mix of anecdotes, and he leaves until last the question some readers will find most tantalising – can birds experience emotions, as humans can?
It would be giving the game away to disclose his conclusions; let's just say that if you pick up Bird Sense, however wise you think you are, you'll learn something new.
Their winter sun break is over and the cuckoos are on their way back
Talking of learning new things: the remarkable revelations from the cuckoo-tracking experiment of the British Trust for Ornithology continue. The BTO, you may remember, last summer fitted five cuckoos in Norfolk with satellite transmitters, and these five birds – Kasper, Martin, Lyster, Chris and Clement – have been followed on their journeys to their wintering grounds in the Congo (nobody had any idea they went there).
Now they're on their way back to Britain, to bring our spring with their cuckoo calls, but rather than head due north towards the Sahara, all five have set off independently to the north-west and the West African rainforest. To stock up for the journey, presumably. "It must be hard-wired into their genes," says the scientist running the project, Chris Hewson.
Martin and Kasper are in Ghana, Chris is in Togo, Lyster is in Nigeria and Clement is in Cameroon. Another month and they'll be here.