Flying over the Himalayas

Image of bar headed geese in flight from John Downer/Nature Picture Library/Corbis

I listened to a really interesting story on NPR this morning about new discoveries regarding the flight patterns of bar-headed geese. These birds are known for their incredible ability to fly over the Himalayas on their annual migration to central Asia. Until now, it was often assumed that the birds reached a specific altitude and then simply maintained it for the duration of their migratory route. A new study published on Thursday in Science questioned this assumption.

Dr. Doug Altshuler (Zoologist, University of British Columbia, Vancouver), who was not involved in the study, was quoted on NPR saying, "It was thought for a long time that they might actually be flying over the peak of Everest itself. Some of the records of early climbers on Everest claimed that when they were climbing on Everest they actually saw the bar-headed geese flying overhead."

Lead study author Dr. Charles Bishop (Biologist, Bangor University, United Kingdom) wanted to understand whether the birds really do remain at one elevation throughout their flight in addition to what impact this annual migration has on the physiology of the birds. So he and his team traveled to Mongolia where they captured molting geese (a period when the birds are unable to fly). Each bird was implanted with a small tube containing monitors that measured altitude, wing-beat frequency, as well as heart rate. The following year they returned to the lake and successfully recaptured some of the same birds from which they extracted the tubes.

The data showed that the birds do not in fact remain at the same elevation during migration. Instead, Dr. Bishop said, "They climb, get over an obstacle, and then go back down again. They just seem to be tracking the terrain, give or take a few hundred meters." Their findings further showed that as the altitude increases, so do the costs of flying resulting in increased heart rate and the birds have to flap their wings harder to create enough lift to keep them in the thin air. Therefore, flying close to the ground simply allows the birds birds to conserve energy and take advantage of occasional updrafts

Fascinating!

Sources:

NPR: Listen to story

Bishop CM, Spivey RJ, Hawkes LA, Batbayar N, Chua B, Frappell PB, Milsom WK, Natsagdorj T, Newman SH, Scott GR, Takekawa JY, Wikelski M, Butler PJ. The roller coaster flight strategy of bar-headed geese conserves energy during Himalayan migrations. 347(6219): 250-254, 2015. DOI: 10.1126/science.1258732