23 February 2014

Building the mosaic

The human retina has two sorts of photoreceptors. Rods (so called for their shape) respond only to dim, monochromatic light, while cones are adapted to the perception of colour and detail. The middle of the retina is formed into a small pit, the fovea. Here the cones are densest; and in the middle of the fovea is an area about a third of a millimetre in diameter, called the foveola, where the concentration of cones is greatest of all.

Rather than perceiving something in detail all at once, we scan it. The musculature of the eyes makes a series of tiny jerks, termed saccades, shifting the foveola from one point to another. In this way the brain builds up a mosaic which the imagination and memory try to make sense of.

Hearing operates in an analogous manner, using a granular series of pressure waves. In fact the whole of our perception works like this.

What we do with the mosaic depends on who we are. Our genetics, prejudices and past experiences are all brought to bear when integrating and interpreting information.

Yesterday I walked the path fringing the harbour. The tidal surge that recently wreaked havoc on this coast has left behind huge quantities of brash – the brown and decaying remnants of marsh plants, mixed here and there with branches of seablite or gorse, driftwood, plastic debris, gates, fencing, boats large and small, even an errant, crazily-angled footbridge. The path is still muddy, but under a mild south-westerly breeze and an even milder February sun it was beginning to dry out. Spring is returning to this hemisphere. The birds know it; but the season is still winter.

The sky was azure, the clouds white, the sea ultramarine. Far away across the estuary, on the sands at the end of the shingle spit, many seals were basking. The profile of the bottom there forms a trap for fish. Those mobile specks, too distant to identify, were fish-eating birds: mergansers, goldeneyes, Slavonian grebes, red-throated divers, perhaps black-throated and great northern divers as well (all were reported later). The scene was alive with birds, a thousand or more brent geese, innumerable gulls, cormorants, swirling clouds of waders – knot and dunlin, mainly, no doubt. For me, with my history and interests, an arc of excitement was building.

At length the path brought me to the fen, a wide, quiet, reed-fringed lagoon behind the sea wall, with inlets and spits where birds can hide, rest and preen. On my preceding visit a single avocet, an early harbinger of spring, had been swimming there and upending like a duck. Now there were forty. The lapwing flock had dwindled; the golden plover had gone altogether, and in their place were black-tailed godwits.

Because I had left my telescope at home, I spent a long time scrutinizing the fen with binoculars only. It is surprising how much information the foveolas can glean, even at low magnification. I sorted through the geese on the far side, admired the pintails (surely there is no more elegant creature than a drake pintail), examined the many black-headed gulls for something rare, counted the godwits and avocets, jotted notes. Just when I thought I had covered everything I picked up a single drake pochard, feeding where the water is deepest.

Descending the wooden steps, I gained the footpath that skirts the western side: I had decided to walk even further, to check the freshwater meres near the coast road. The footpath is thickly lined with scrub and small trees, but here and there affords views of the fen.

My afternoon was assembling grain by grain, like the pixels on this computer screen or the frames of a cinema film, like the words of a story or even the letters of a sentence. I was content, more than content, with my reward for the long walk. This landscape has always suited me very well, and if the day had offered nothing more I should have gone home happy.

I stopped once more to raise my binoculars to the fen. They were filled with leafless willows, a tawny wall of dead reeds, gunmetal water, all sunlit from the right, the colours perfect. Then, low above the reeds, I saw the thuggish bulk of a female sparrowhawk coming slantwise in my direction. Her brown plumage co-ordinated exactly with the waxen twigs and branches of the willows. In an instant she was lost to view behind the foreground hawthorns.

A bird of prey is a solitary assassin, living fast, much of its time yielded to the imperative to feed. An evolutionary arms race joins predator and prey: most attacks end in failure. As the winter afternoon wanes a hungry hawk can become desperate and even reckless.

Sparrowhawks hunt in various ways. A common ploy is to cruise the length of a hedge and flip over it to surprise whatever is on the other side – a blackbird or even a wood pigeon, for a really strong female hawk. This one was using the reedbed like that.

For an odd, floating moment I was the only one who knew she had arrived. Then most of the birds on the fen took flight, many more lapwings than I had been able to count, avocets, gulls, even some of the teal and wigeon; the rasping cries of a fleeing snipe came from high above the willows.

I could not tell whether the hawk had struck, and didn’t see her again, but my glimpse of her had unified what had gone before. It was the final piece of mosaic. The naturalist longs to blend with nature, to learn its secrets, a hopeless quest but one that always draws him on. Just for those two or three seconds before the fen erupted, I was as much hawk as human.

It bears saying that each of has unique experiences, all the time. Each of us is building a unique vision of the world. If I had no interest in birds, or had simply been looking the other way, my afternoon would have felt quite different. And your past few minutes would have felt different too, because you would not have been reading this, but assembling some other mosaic, in keeping with the larger mosaic that conforms to the unique combination made by your genetics, history, and system of beliefs.

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