Nesting swallows give dazzling aerodynamic display

ANOTHER LIFE:THE SWALLOWS took a long time to find us, deciding only a few years ago that our open-sided woodshed was enough of a barn to nest in.

Their mud-cups grew on a ledge in the darkest corner. This spring, however, they moved house again, discovering that a missing batten would give them entrance to a proper cave – the shed next door where we hide the car from salty winds.

The birds hurl themselves through this two-inch letterbox some hundreds of times a day to feed pellets of midges to their young. There are only three chicks (a modest, beginners’ brood) but they gape insatiably, a row of yellow throats between tin roof and adobe saucer of a nest. Picking my moments, I creep into the car to watch.

At something over two weeks old and beginning to shake and stretch their wings, the chicks need service from both parents. There are round estimates of feeds –­ 400 visits from each parent from dawn to dusk, 600 flying miles, that sort of thing –­ but the tempo varies greatly. The adults have to nourish themselves, for one thing, so nest visits slow down markedly around the middle of the day.

Afternoon brings a steady crescendo of feeds­– many at half-minute intervals, others only seconds apart in near-simultaneous swoops through the slot of the missing batten. This may be tied to peak fly supply, but there’s enough protein, anyway, for the chicks to have taken on their own sanitation, ejecting white, faecal balloons over the edge of the nest.

Whether I get to see the episode where they climb out of it and their parents tease them with food into lurching into the air, will be largely a matter of luck: I have lost track of the crucial 21st day.

Meanwhile, we stand at the kitchen window at tea-time, mesmerised by the swallows’ aerobatics.

Between window and road they have a sheltered hunting ground, sheds on one side, trees on the other, through which they weave their astonishing loops. “The swallows twisting here and there/ Round unseen corners of the air” was how Scottish poet Andrew Young saw them, but our birds have solid corners to evade, not to mention their occasional, breathtaking swoops beneath the garden gate.

Their last-second Immelman turns at the window have brought us dazzling close-ups of wings in shimmering indigo, of crimson face and throat, and also of the one body feature that tells male from female in the European barn swallow: the different length of the streamers, outermost feathers of the tail. On average, the male has an extra 15 millimetres (around five-eighths of an inch).

THE FUNCTION AND significance of this little margin has long been debated among evolutionary biologists. Is the extra length in the male the product of natural selection, refining his manoeuvres in flight, or of sexual selection for reproductive success? In the peacock – a classic example, if hardly of flying ability – the male’s gorgeously exaggerated tail is unmistakably part of competition for a mate. It seems, indeed, that positive genetic feedback links both female preference and the exaggerated trait.

In barn swallows, too, as shown by decades of studies, male streamer length seems to figure in reproductive success – males with shorter streamers do less well in finding mates than their better-endowed fellows.

Indeed, giving male swallows a bit of extra length in their streamers has been shown to improve their chances, not only of finding and holding mates but of the odd bit on the side.

But the streamers obviously count, for good or ill, in the way barn swallows fly. Aerodynamic theory suggests that only the spread, triangular tail increases lift, while long streamers beyond it should increase the tail drag and thus cost more in energy. Male swallows have been filmed dodging obstacles in wind tunnels, with their streamers cut short, or added to. Some have argued that the longer streamers, twisting the feathers in their sockets, actually reduce drag by feathering the tail’s leading edge, allowing tighter turns in pursuit of prey.

Discussion and research go on. Current consensus seems to allow some extra length in the male’s streamers from natural selection, with a little bit more for sexual show. I am reminded of the great debate on whether the colossal spread of antlers on the ancient Irish elk evolved as status symbols in ritual confrontations between males, rather than working weapons in their battles for a harem of hinds. Horn-to-horn jousts on the roof of Dublin’s National Museum were part of practical research to show that the antlers did function very effectively, but climate change hastened the giant deer’s extinction through the energy cost of growing them.

Long may Hirundorustica gorge upon our midges and swoop beneath our garden gate.