Tuesday, September 18, 2012

Unflappable albatrosses

ResearchBlogging.orgWandering albatrosses, Diomedea exulans, are among the largest flying birds in the world and are renowned for soaring flights of thousands of kilometers to feed. Several adaptations allow their flight to be extremely energy-efficient. For instance, their extremely long wings allow them to glide remarkably long distances and a modified tendon allows them to hold their wings open without the use of their muscles.

A wandering albatross showing its fight position (photo Wikipedia)
For a long time, it's been clear that albatrosses are using wind energy to power their flight. Indeed, Lord Rayleigh proposed that albatrosses were using wind shear to soar in 1883. Although other mechanisms have been proposed, dynamic soaring in wind shear has since been cited as the principle mechanism that they are able to gain energy from the wind. 

At the ocean surface the wind travels more slowly because of friction, but as you move away from the surface wind speeds get higher. Albatrosses can gain airspeed by rising away from the sea into the faster winds and then dropping back into the slower winds at the surface. They first turn into the wind and rise followed by a turn with the wind as they descend, gaining energy in both directions while losing some to drag.

The dynamic soaring cycle of an albatross. It starts with a turn into the wind, then a climb in altitude, then a turn with the wind and a descent back to the sea surface. The length of yellow arrows to the left of the figure indicate the strength of the wind at different altitudes (Image taken from Sachs 2005).
Penncuick argued that albatrosses couldn't get enough energy from the wind-gradient and must be deriving a large amount of energy from moving in and out of the pockets of almost still air in the lee of wave crests, which he termed 'gust soaring'. Other authors have suggested that they slope-soar off the windward side of wave crests. But, the debate over how albatrosses are gaining enough energy for their long distance flights has played out in the theoretical literature, sometimes accompanied by anecdotal observations of flight behaviour.

The gust soaring cycle of an albatross. As the albatross moves through the 'separated boundary layer' (blue line) from the leeward side of a wave crest it gets a kick of energy from the wind allowing it to gain altitude and potential energy to power its soaring flight (Image taken from Richardson 2011)
With their paper published recently in PLOS One, Sachs et al. have added some empirical data to help resolve the issue. They attached small GPS devices to the backs of 16 albatrosses, which measured the position and altitude of each individual once every second and the velocity 10 times a second. This allowed them to look at the small scale of the flight cycle and draw inferences about the physics of the manoeuver.

A plot of a recorded dynamic soaring cycle. The numbers indicate each stages from the ascent [1], to the turn at peak altitude [2], to the descent [3] and the turn to restart the cycle [4] (Image taken from Sachs et al. 2012).
They used the data to calculate the total energy over the entire dynamic soaring cycle by summing the potential and kinetic energy. Contrary to the expectations of gust soaring and slope soaring, the maximum energy in the cycle was reached on the descent. And the energy accumulation was gradual, without any large spikes that would result from a big kick in energy close to the surface.
A two-dimensional plot of the soaring cycle showing the point at which maximum and minimum energy are reached. The track shown here is the same as the one above (Image taken from Sachs et al. 2012).
Sachs et al. also calculated the energy gain that the albatrosses could achieve throughout the cycle. The maximum energy in the cycle was ~360% of the minimum energy and provided enough surplus to overcome drag forces. Indeed, the energy gain was so large that it far exceeded what the albatrosses could achieve by flapping their wings.

The efficiency that the albatrosses converted the wind into usable energy for flight allowed them to achieve ground speeds higher than the wind speed. On average the 16 birds that they followed traveled at ~60 kilometers per hour, but one bird was clocked traveling at an average of 76 kilometers per hour. If that's not amazing enough, an earlier study that Sachs et al. cite, clocked a grey-headed albatross (Thalassarche chrysostoma) traveling at an average ground speed of 110 kilometers per hour for 9 hours in high winds! 

My only concern about the paper, which isn't a very big one, is that all of the energy calculations are based on a single dynamic soaring cycle (the one in the two figures above). The authors do present three others in their supplementary material, which all look essentially the same. But, I wonder why they don't use them for the calculations. And they tracked 16 birds for at least 176 kilometers, did they really only get four cycles which occur in the space of ~150 meters? They don't say.

References:

Sachs G, Traugott J, Nesterova AP, Dell'omo G, K├╝mmeth F, Heidrich W, Vyssotski AL, & Bonadonna F (2012). Flying at no mechanical energy cost: disclosing the secret of wandering albatrosses. PloS one, 7 (9) PMID: 22957014  

Richardson PL (2011). How do albatrosses fly around the world without flapping their wings? Progress in Oceanography, 88 (1 - 4), 46-58 DOI: 10.1016/j.pocean.2010.08.001  

Pennycuick CJ (2002). Gust soaring as a basis for the flight of petrels and albatrosses (Procellariiformes) Avian Science, 2 (1), 1-12

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