The paradox of extreme high-altitude migration in bar-headed geese Anser indicus

Abstract

Bar-headed geese are renowned for migratory flights at extremely high altitudes over the world's tallest mountains, the Himalayas, where partial pressure of oxygen is dramatically reduced while flight costs, in terms of rate of oxygen consumption, are greatly increased. Such a mismatch is paradoxical, and it is not clear why geese might fly higher than is absolutely necessary. In addition, direct empirical measurements of high-altitude flight are lacking. We test whether migrating bar-headed geese actually minimize flight altitude and make use of favourable winds to reduce flight costs. By tracking 91 geese, we show that these birds typically travel through the valleys of the Himalayas and not over the summits. We report maximum flight altitudes of 7290 m and 6540 m for southbound and northbound geese, respectively, but with 95 per cent of locations received from less than 5489 m. Geese travelled along a route that was 112 km longer than the great circle (shortest distance) route, with transit ground speeds suggesting that they rarely profited from tailwinds. Bar-headed geese from these eastern populations generally travel only as high as the terrain beneath them dictates and rarely in profitable winds. Nevertheless, their migration represents an enormous challenge in conditions where humans and other mammals are only able to operate at levels well below their sea-level maxima.

Figure 1.

Satellite tracks of migrating bar-headed geese: (a) three-dimensional map showing release locations (black crosses) of bar-headed geese in India (n = 2 sites) and Mongolia (n = 1 site). Coloured lines represent 16 individual geese, and coloured background shading indicates elevation. Solid thick white line shows the great circle route. White crosses show locations of the 14 ‘eight-thousanders’ (the world's highest mountains, each over 8000 m in elevation). Five peaks are largely obscured owing to their proximity to other peaks. (b) Cross-section of land elevation under the arithmetic mean bar-headed goose northwards migration from Indian wintering (left side of plot) to Chinese and Mongolian breeding grounds (right side of plot).

Figure 2.

Timing and altitude of locations: histograms showing relative frequencies (densities) of (a,b) timing of locations and (c,d) altitudes for (a,c) migrating and (b,d) stationary bar-headed geese (e.g. during stop-over events, breeding and wintering).

Figure 3.

Bar-headed goose migration and altitude: map showing GPS locations received from bar-headed geese (black dots) on the Tibetan Plateau (the Himalayan mountains are in northern Nepal near the southern Chinese border). Background greyscale shading shows elevation from SRTM, and red shading shows elevations (a) above 5000 m, (b) above 5500 m and (c) above 6000 m. Left column shows migration across Tibetan Plateau. Middle and right columns show zoomed extents of commonly used valleys. Extents shown as black boxes.

Figure 4.

Bar-headed goose flight ground speeds. (a) Frequency histogram of minimum transit speeds received during migration. Vertical dashed lines show V_MP (minimum power speed) and V_MR (maximum range speed, the most efficient speed per unit of distance) at sea level. (b) Scatter plot of instantaneous GPS ground speed and minimum transit speeds. Black line shows line of equality, where geese could not have stopped between transmitted locations. (c) Box plot showing median instantaneous ground speeds in 1000 m altitudinal bins (comprising median of median values per goose). Black line shows median value, box shows inter-quartile range, dashed line shows total range. Number of locations for each altitudinal band indicated.

Figure 5.

Winds experienced by migrating bar-headed geese: wind patterns (mean wind speed and direction) during (a) southward migration (1 October–31 December) and (b) northward migration (1 March–31 May), where arrows indicate wind speed and direction (largest arrow size and red colour denotes strongest wind speeds; see legend). Black arrow shows schematic of travel. Black dots show GPS locations received from bar-headed geese at coincident temporal scale to winds.