Long before any jumbo jets had taken to the air, albatrosses were navigating the global skies, travelling up to 15,200 km in any one trip. However, undertaking such long journeys requires a lot of energy, and calculations suggest that albatrosses cannot rely on flapping their wings unless they sacrifice half of their body mass as fuel. To overcome this problem, albatrosses have learnt to harness the energy from the winds just above the sea's surface. But how exactly do they do this? Some researchers have suggested they use gusts of winds caused by breaking waves and others think that using the shear wind gradient, where wind speed increases with altitude, may be sufficient. With so many theories, Gottfried Sachs, from the Technische Universität München, Germany, decided to investigate in more detail with the help of his aerospace engineering background (p. 4222).
As GPS technology has become much smaller and more advanced in recent years, Sachs and his PhD student at the time, Johannes Traugott, realised they could use GPS loggers to investigate the albatrosses' movements in fine detail. By manipulating the loggers to increase data recording to 10 times per second and developing a special computational algorithm, Sachs and Traugott could track both horizontal and vertical movements to within decimeters. Equipped with 20 GPS loggers, Traugott then made the long journey to the remote Kerguelen Islands in the Indian Ocean. With the help of biologist Anna Nesterova, a post-doc in Francesco Bonadonna's lab at the CNRS Centre d'Ecologie Fonctionnelle et Evolutive, France, he was then able to attach his loggers to albatrosses just about to depart the islands on a long foraging trip.
From the logged data, the team could then characterise the small-scale soaring and diving movements the birds made, into four distinct phases: (1) a climbing upward phase against the wind, (2) a leeward turn, (3) a downward descent with the wind and (4) a windward turn just above the sea to reorient themselves against the wind for their next climb. The GPS data also provided the team with information of the albatrosses' speed and altitude so they could calculate where and exactly how most of the energy was gained. While some energy was gained because of an increase in altitude, most of the harnessed energy was kinetic energy, gained after the albatrosses had made their leeward turn and were heading downwards with the wind behind them.
With some further analysis, Sachs found that albatrosses will climb to different altitude levels: in one case this was 9 m above sea level and in another it was 15 m, suggesting that they will fly high enough to enable them to the gain sufficient energy to sustain continuous non-flapping flight. Next, Sachs calculated the maximum propulsive force generated from this wind and found that it was more than 10 times higher than anything the albatross could create by merely flapping its wings. This conclusively showed, for the first time, that at no point during their four-step routine do the albatrosses resort to this energy-draining mode of flying.
What's more, with his calculations, Sachs was also able to rule out a number of other theories. He shows that the energy gain cannot be explained by the wind gradient alone, which predicts that energy would be gained on the upward stage of flight. He also ruled out that gusts caused by crashing waves helped, as several albatrosses started their acrobatic manoeuvres whilst still over land and there were no sudden gains in energy. All in all, the albatrosses seem to have mastered a very complicated flight manoeuvre that allows them to fly for ‘free’.
