Another Life: An invisible chorus above my window

A beginners’ brood of swallows has emerged from the woodshed

Swallows: master and mistress of the garden. Illustration: Michael Viney

Swallows: master and mistress of the garden. Illustration: Michael Viney

Sat, Aug 2, 2014, 01:00

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.

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