TOUR de France cyclists exploit the slipstreams of their competitors in ‘le peloton’.
By ploughing a channel through the air, a leader makes it easier for the one behind him. Large birds fly in V formation for the same reason. However, according to a paper just published in Nature, there’s more to formation flying than mere jostling for position.
The northern bald ibis is a large, black bird with a long, pink bill, curved downwards like a curlew’s. It once bred throughout southern Europe, as far north as Germany. Numbers began falling in the 16th century and have continued to do so ever since; excessive hunting and habitat destruction are blamed. Until recently, there were colonies in Morocco, Algeria, Turkey and Syria. Only the Moroccan ones remain.
Bald ibises breed well in zoos, including Dublin’s. In 2003, aviaries were established in Andalusia and Austria. Zoo-bred birds were allowed to fly freely and some are now living wild. There’s a problem, however, at migration time; the birds don’t know where to go. It’s thought that the Austrian ibises, of long ago, moved to the south of Italy for the winter. Youngsters learned the migration route by accompanying their elders, but the reintroduced birds have no-one to teach them.
Trumpeter swans, reintroduced in the southern United States, had the same problem. Not knowing where to go at migration time, the youngsters headed off in all directions, many of them coming to grief. American researchers adopted a novel solution; they trained young swans to follow a micro-light aircraft. Imprinted on the pilot, who wore a special suit, the swans were led, stage by stage, along the migration route. This ‘come fly with me’ approach worked, but there were difficulties: swans are aggressive. Males jostle to be ‘top gun’, and don’t take easily to being upstaged. Disgruntled birds sometimes sulked and left the formation. Flying had to be suspended until the prodigals were persuaded to return to the fold.
Ibises are more laid-back and cooperative. They even agreed to wear lightweight harnesses, to which little heart-monitors and motion sensors were attached. Fourteen ibises, a bird equivalent of the Red Arrow team, were trained to fly behind a propeller-driven parachute, supporting their handler and the pilot. These youngsters had never seen a V formation. Would they know how to form one, the researchers wondered? They need not have worried; the trainees arranged themselves in the traditional V behind the para-plane. Formation flying in ibises is instinctive rather than learned. Wing-beats generate waves. Each bird positions itself a little over a metre from the one ahead of it, and at an angle of around 45 degrees to the direction of travel. Aerodynamic analysis shows that this is where the updraft from the preceding bird’s wings is strongest. Not surprisingly, birds flying in this position had reduced heart rates. But refinements to the system astonished the scientists. To make best use of the energy available, a bird monitors the wing-beat of the one in front and synchronises its own wing-beat to it. In what the researchers refer to as ‘wingtip path coherence’, it keeps its wingtip in the path of the one ahead. Whether birds do so by sight, or by detecting pressure changes in the air, isn’t clear.
The density of air probably rules out V formation flying in small birds, explaining why it’s practised mostly by birds above a certain size. Brent, our smallest geese, only use it sporadically. Birds smaller than Brent, with a few exceptions, seldom fly in a V. The bald ibises were led by a circuitous route to Tuscany. It took three weeks. They don’t have to be shown the way back to Austria; they will remember what the para-plane pilot, their surrogate parent, taught them.
Trained birds will, it’s hoped, teach future generations of ibises where to go at migration time.
* ‘Upwash Exploitation and Downwash Avoidance by Flap Phasing in Ibis Formation Flight’. Stephen Portugal et al. Nature. 2014.
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