The trans-Himalayan flights of bar-headed geese (Anser indicus)

Abstract

Birds that fly over mountain barriers must be capable of meeting the increased energetic cost of climbing in low-density air, even though less oxygen may be available to support their metabolism. This challenge is magnified by the reduction in maximum sustained climbing rates in large birds. Bar-headed geese (Anser indicus) make one of the highest and most iconic transmountain migrations in the world. We show that those populations of geese that winter at sea level in India are capable of passing over the Himalayas in 1 d, typically climbing between 4,000 and 6,000 m in 7-8 h. Surprisingly, these birds do not rely on the assistance of upslope tailwinds that usually occur during the day and can support minimum climb rates of 0.8-2.2 km·h(-1), even in the relative stillness of the night. They appear to strategically avoid higher speed winds during the afternoon, thus maximizing safety and control during flight. It would seem, therefore, that bar-headed geese are capable of sustained climbing flight over the passes of the Himalaya under their own aerobic power.

Fig. 1.

Timing of migrations with 30-min average wind speed and direction from the Nepal Climate Observatory at Pyramid station for geese migrating (A) northward (n = 8) and (D) southward (n = 12) over the Himalaya. Arrows show cardinal direction (north pointing up to 0 °) in which the wind was blowing and arrow length (indicated in A) is proportional to wind speed in A and D. (B) Map showing the northward migration routes; weather station (WS) location is indicated. (C) Elevation of the mean northward track across the Himalaya (for all crossing locations from all eight geese), blue circles show individual data points and blue line shows Lowess smoother for mean ground elevation under the track (black line).