Sounds like Springtime

For all the spring abundance of birds on the acre, the pouring out of song and hectic mating chases, I seem to be very bad at spotting nests. In winter, when the leaves fall, I see where I should have been looking: a wren's ball of moss in the thorn hedge outside the door; a dunnock's cup in a bush I had brushed past 100 times.

This spring, I can see where the song thrush has been stripping off rotten wood from the logs around the garlic bed, but have to imagine where she has been laying down her plywood floor with fibres and saliva. I catch the blackbird ducking back under her bush, but don't get down on hands and knees to peer up its skirts.

Last week, however, a passing glance into the willow outside the living-room window caught something that did not belong in the web of bare branches. The tree grows below the window and its boughs curve like the ribs of a boat or basket. Cradled in a classic, three-way fork was this big, soft doughnut in silver-green, nearly the shade of the willow.

Even as I watched, its builder arrived with a wisp of moss in her bill and stabbed it into the wall of the nest. Then she settled inside it with her plumage fluffed up and began to rotate, slowly, anti-clockwise, pushing down with her breast and jerking up her tail. She was sculpting the nest to a comfortable fit.

The female chaffinch is a rather dull little bird compared with the cock's almost parakeet brilliance, but to watch her in the nest is to guess why. Its outer layer is woven of lichens and spider-silk; the next of mosses and grass; the innermost, a cup of interlaced feathers and fine roots. Amid all these greys and greens, she can close her eyes and disappear.

The nest had taken several days to build - a week, if I could be so blind. On average, say the books, the hen makes about 1,300 visits at something up to 20 per hour: that seems about right. Between gathering a few more wisps and snacking at our peanuts, she is off playing chasing games with the cock: they rocket through the trees like guided missiles. In one of her long absences, I watched a passing great tit perch for a moment on the rim of the nest, examine it enviously, then fly on.

I worry a bit about her choice of site. Although it is splendidly sheltered and secure, if I stand on a chair inside my window, I can see right into the nest, and a magpie on the ESB pole could do the same (there are four of these bandits teetering on a spruce tree down the acre, white bellies flashing against the sea). It will be a race between the chaffinch getting her eggs laid and the willow sprouting enough catkins and leaves to hide them.

The great tit, on the other hand, is safely installed in a hole somewhere, shaping a cup in a mass of moss and lining it with sheep's wool plucked off briars and blackthorn.

This turning round and round in the nest is common to most birds, whether they nest in trees or on the ground. Even a blue tit in a nesting-box rubs her breast against the floor in the same way - this before she has even begun to gather nesting material.

The evolutionary explanation is that nest-building in birds goes back to pits scraped out in the ground by their reptile ancestors - themselves the inventors of the terrestrial egg. Even today, there are Australian birds which bury their eggs in the ground and let the sun's warmth do the incubating.

Evolution to warm-bloodedness gave birds the capacity to hatch eggs out beneath their bodies. As they developed flight, they could disperse to cooler climates and still reproduce. Nesting in trees was often safer than on the ground, but the habit of scraping and pressing out a hollow seems to have remained part of the genetic D-I-Y kit. A dish in the earth, or bowl in the sky, it is still the best shape for keeping eggs rolled together.

The nesting season has scattered the acre's dozen resident house-sparrows - though not, I am sure, very far. I miss their noisy afternoon gatherings, in the thorn bush over the septic tank, for what has been described as "social singing". Instead, the male sparrow's chirrup has speeded up into an urgent, rhythmic rapture, which is as near as a sparrow gets to actual song.

When the sparrows arrived on the acre a year or two ago, a ripple of city-boy racism stirred in my heart. Did we really want common sparrows rubbing wings at the peanut-feeders with the glorious finches, green and gold, and the technicolor tits?

Since we seemed to have little option, I decided to change my attitude. Country house sparrows are really quite smart in their way, even ethnically and culturally interesting. A century ago, after all, they were the abundant bird of the thatched, Irish-speaking clachans of the west, especially in the "congested districts" of Co Donegal and west Co Mayo. There was a large colony of sparrows nesting in the ivy-covered cliff beside the harbour at Inishturk, for example, and plenty around even the most exposed cabins of Tory and Aran.

Sparrows are basically seed-eaters and flourished on the oat-fields, hay-stacks and spilt grain of the countryside - all now disappeared. In the towns, too, the passing of the horse cut the sparrows' food supply. Just as significantly, the drop in city insects in recent decades has meant less food for the sparrows' nestlings in spring.

Birdwatchers in Dublin, Belfast and London have remarked on the scarcity of sparrows in the inner city. For Dubliners, here's something to check (from John Watters' Natural History of the Birds of Ireland, published in 1853): "The sparrow conveys an excellent idea of the ludicrous, from the sites chosen to nidify (nest) in, as we have seen the bulls' heads which grace the entablature around the Rotundo in Dublin, with the hollowed eye-sockets filled up, and a joyful young brood twittering away merrily in both."

If the bulls' heads are still there, are the sparrows?