How the bar-headed geese, a 3 kg bird affront their migration across the Himalayan range?
The big question that was motivation of several documentaries was finally answered in a paper recently published in Science.
Twice a year, around 100.000 bar-headed geese migrate from Mongolia to the Indian coast. A trip of approximately 4.500 km that crosses the Himalayan range. Researchers discovered that the geese prefer to fly during the night as low as possible and without needing help from the wind. The animals do not stop beating their wings and can reach as high as 5.000 meters over sea level (flying over the 5th highest peak in the world) and descend 1.000 meters in few minutes to ascend again. The scientists compared their trip to a aerial roller coaster ride.
On average, the geese flew at an altitude of about 4500 meters, but often they changed altitude. For example, one of the four birds whose data was retrieved dropped 1000 meters in 20 minutes, then climbed more than 2000 meters in the next 1.5 hours. Yet, one bird has been tracked migrating as high as 7290 meters, an altitude where many humans would require supplemental oxygen to move.
As part of a large international research project, led by Bangor University and primarily supported by the BBSRC (grant BB/F015615/1) and the Max Planck Institute, bar-headed geese have now been GPS tracked (Hawkes et al. 2011 PNAS http://www.pnas.org/content/early/2011/05/27/1017295108 and available here for download) flying over the Himalaya and data reveal that they do not normally fly higher than 6,300 m elevation, flying through the Himalayan passes rather than over the peaks of the mountains. This altitude is clearly still very impressive (equivalent to Everest Camp II) and challenges even the fittest human climbers. Without proper acclimatisation to such altitudes, mammals suffer from high altitude related sickness very quickly, and in some cases this can be fatal.
The physiological and biomechanical requirements of flight at high altitude have been the subject of much interest. Here, we uncover a steep relation between heart rate and wingbeat frequency (raised to the exponent 3.5) and estimated metabolic power and wingbeat frequency (exponent 7) of migratory bar-headed geese. Flight costs increase more rapidly than anticipated as air density declines, which overturns prevailing expectations that this species should maintain high-altitude flight when traversing the Himalayas. Instead, a “roller coaster” strategy, of tracking the underlying terrain and discarding large altitude gains only to recoup them later in the flight with occasional benefits from orographic lift, is shown to be energetically advantageous for flights over the Himalayas.
Source and access to the original paper: http://www.sciencemag.org/content/347/6219/250
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