We needed a few bright logs from the woodshed to take the bleak look off the cold summer stove – would the swallows mind? They’ve had the shed to themselves these past few weeks, plastering this year’s nest to a roof beam and swooping in and out to attend their brood. They’ve been master and mistress of the way to the gate, swishing back and forth from dawn to dusk to scythe invisible midges.
With a muttered “Never mind me”, I stooped to find a likely grip on timber, whereupon a dark explosion erupted above my head. The fledglings fanned out in panic, then wheeled above the trees in what I fondly choose to think was their very first flight. There were now too many swallows carving the air to be sure how many swallows there were.
Eventually the young ones settled, perching in a row on the roof gutter above my bedroom window: an invisible chorus of twittering, rising and falling between parental feeds. I crept out to count and scored four before they flew. There might have been three, the modest beginners’ brood, or up to half a dozen. Brood size is partly matched to insect abundance – swallow species in Europe lay more eggs than those in the tropics, where shorter summer days give fewer feeding hours.
But I have been learning about another, subtle organ of control: the brood patch. Kitchen window birdwatchers will have noticed, perhaps, a cleft in the breast feathers of, say, a motherly thrush, out taking a break on a branch – may know, indeed, that it marks the patch of bare skin which keeps the eggs warm beneath her in the nest.
That it does much more is explored in the chapter on "touch" in Tim Birkhead's excellent book Bird Sense (Bloomsbury, 2012). The egg is warmed by the flow of blood to the skin, and this is under the bird's regulation. But contact between egg and brood patch also triggers the release of the hormone prolactin from the pituitary gland beneath her brain and this, in turn, keeps her incubating until her clutch of eggs is completed.
What happens if the eggs are removed can be remarkable and was first tested in an experiment by the English naturalist Martin Lister as long ago as the 1670s. As a swallow laid her eggs, Lister removed them one after another, whereupon the bird continued to lay no fewer than 19 before stopping. There have been other such heartless if illuminating interventions – the sparrow that laid 50 eggs instead of the usual four or five, and an American northern flicker that reached 71 over 73 days. But some birds – the lapwing is one – are immune to such interference and just lay the number of eggs their species is used to. Ornithologists now divide birds into determinate and indeterminate egg-layers, but still don't know the rationale.
The day of the young swallows seemed to fill the garden with little birds: great tits perched in series up a thistle; blue tits tapping our windows for spiders; willow warblers tip-toeing over the fronds of a conifer. This was all very delightful, but could not last. Young birds succumb to the wanderlust that science calls dispersal, while the older ones hide in the shadows to change their feathers for the winter. August brings a different mood to the garden and, without the promptings of territory or hunger, a slowly relaxing silence.
All birds moult, but in different ways. Small birds can’t afford to lose their speed and flight-power. They change their feathers a few at a time, new ones pushing the old ones out. The starling, for example, started shedding its main flight feathers in late June, and the total replacement of its plumage takes about 100 days. The young birds in the little summer flocks, zooming around the hillside in quite alarming, hedge-hopping flight, seem too dark to be starlings at all. But by springtime the tips of their feathers will have worn away, leaving the sleek, iridescent, bright-speckled starlings that we know.
Young blackbirds, too, have begun to moult into adult plumage, changing all their feathers except those of the tail and some in the wings. The results can confuse the human eye, some of the males almost as black and glossy as their fathers; others, in this first year, almost as brown as their mothers. Adult blackbirds, wandering out on the lawn with a moult half-complete, can look distinctly scruffy and déclassé and fleetingly uncertain of their sex.
I end with how it works for our barnacle geese in Greenland. When the chicks are born and coaxed to whirr down from the cliffs, the family groups march across the tundra to the nearest big lake. There, the parents drop their primary flight feathers. While new ones grow, any glimpse of a fox sends the flock out into the middle, where they float together safely in a raft of black and silver.