Saturday 17 November 2012

How Bar-headed Geese cross the Himalayas.

Bar-headed Geese, Anser indicus, are well known for their annual migration between breeding grounds in Tibet, Mongolia and northern China, and wintering grounds in southern India, a migration route that involves crossing the highest part of the Himalayas. There have been frequent reports of climbers witnessing the Geese at extremely high altitudes, and they are regularly cited in popular literature as achieving sustained flights at altitudes exceeding 8000 m. However empirical data on such flights has been lacking, and migration routes in excess of 8000 m seem highly unlikely to biologists, as the air at this height is rarified (thin), providing little lift to the Geese (which are quite large Birds), and therefore requiring more energy, and at the same time depleted in oxygen, making less energy available. In favor of the high altitude migration route, it has been agued that it would represent a much shorter total journey and that it would reduce the risk of Geese flying into mountains.

Bar-headed Geese (Anser indicus) in flight. Rajiv Lather/Birding in India.

In a paper published in the Proceedings of the Royal Society Series B: Biological Sciences on 31 October 2012, a team of Scientists led by Lucy Hawkes of the School of Biological Sciences at the University of Bangor and the Environment and Sustainability Institute at the University of Exeter, present the results of a study in which Bar-headed Geese were radio-tagged in order to trace their travel routes through or over the Himalayas.

Hawkes et al. found that the Geese spent 95% of their time bellow 5784 m (still impressively high), choosing to take a longer route through the Himalayas in order to utilize lower-altitude valleys and passes. Only 10 of the 91 Geese tagged were ever recorded above this altitude, and only one exceeded 6500 m, reaching 6540 m on an overnight flight, when the air was particularly cool (and therefore dense). While they cannot rule out the possibility that Geese do sometimes reach higher altitudes, Hawkes et al. strongly suspect that tales of Geese flying at 8000 m are apocryphal, owing more to climbers' folk-law than to accurate observation.

(a) Map showing the migration routes of the tagged birds. (b) The land elevation of a cross-section through the migration route (exaggerated). Hawkes et al. (2012).

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