Movement Over the Himalayas.

Our data show that bar-headed geese travel north, from sea level over the Himalayas, in a single day (median date March 24, range March 15 to May 6). The median crossing time was 8 h (n = 5 geese tracked for the complete crossing, Fig. 1 A and B) and the shortest flights were ~7 h long. Minimum 3D transit speeds (median 53.2 km·h⁻¹, calculated from the straight line distance and altitude traveled between successive hourly locations (n = 18 locations) were correlated with instantaneous speeds transmitted by the satellite tags (here after referred to as “tags,” see Methods; median 64.5 km·h⁻¹, Spearman's rho = 0.71, P < 0.01), indicating that the geese were flying aerobically and could not have stopped for prolonged periods (i.e., >15 min) during the climb. These median speeds are close to the predicted minimum power speed (61.2 km·h⁻¹) (8), thus minimizing forward flight costs and maximizing additional power available for climbing. The range of speeds recorded are also similar to those previously published for captive bar-headed geese flying in a wind tunnel (26), suggesting that the geese flew in the absence of strongly assisting tailwinds, which would have increased forward flight speed significantly. Geese did not travel faster in anabatic daytime conditions compared with katabatic nighttime flights (flying at 63.0 versus 59.0 km·h⁻¹, respectively, Wilcoxon test P > 0.05). Flight theory (8) suggests that at higher altitude, where air is less dense, drag (for example profile and parasite drag) is reduced, but the power required to generate lift and thrust increases; minimum speeds required to maintain horizontal flight are faster and, therefore, more expensive. Bar-headed geese in this study did, indeed, travel faster at higher altitude (Wilcoxon test, P < 0.01), with geese traveling at 54 km·h⁻¹ (median value) at lower altitudes compared with 67.0 km·h⁻¹ at higher altitudes.

Geese climbed northward over the Himalaya (Fig. 1 A and B), gaining 1.1 km of altitude per hour (median value, n = 6 geese, range 0.8–2.2 km·h⁻¹, calculated from the 18 steepest climbs reported at successive hourly locations). The three highest climb rates of 1-h duration were 1.88, 1.94, and 2.15 km·h⁻¹, along with transit speeds of 43.2, 45.8, and 68.2 km·h⁻¹, respectively. In addition, three geese averaged climb rates of 1.4, 1.2, and 1.1 km·h⁻¹, along with transit speeds of 31.1, 41.0, and 63.5 km·h⁻¹, respectively, over 3 h. These observations represent the longest continuous climbing rates ever recorded. Climb rates were not affected significantly by altitude, flight speed, or wind conditions (with geese climbing at 1.04 versus 1.40 km·h⁻¹ in anabatic versus katabatic conditions). Geese generally matched the underlying terrain during their climbs (median height 130 m; Fig. 1C) and were most frequently located within 340 m of the ground (containing 80% of locations) and thus avoided climbing any steeper than was necessary. Rates of climb reported for similarly sized Greylag geese (1.15 km·h⁻¹ sustained for 20 min at sea level) (1) were similar to our values. However, bar-headed geese sustained their climb rates over much longer periods and at much higher altitudes.

We also tracked 13 geese during autumn southward flights into India, after breeding in Mongolia (median crossing date November 20, range November 10 to December 19). Bar-headed geese cross the Himalaya southward in 4.5 h (median value, n = 8 geese), and in as little as 3 h (n = 2 geese). Geese traveled significantly faster than the predicted minimum power speed (Mann–Whitney U = 108, P < 0 0.05) but on average, neither their descent rate (median 1.3 km of elevation per hour, range 0.9–4.1 km·h⁻¹) nor their speed (64 km·h⁻¹) were significantly steeper than their 18 steepest climb rates and speed over the Himalaya when going northward (descent rate: Mann–Whitney U = 93, P > 0.05, speed: U = 481, P > 0.05, excluding stopovers). Southward migrating geese neither traveled nor descended significantly faster in anabatic versus katabatic wind conditions. However, instantaneous speeds transmitted from geese flying southward (median 64.0 km·h⁻¹) were significantly faster than minimum transit speeds (mean 55.8 km·h⁻¹, Wilcoxon test, P < 0.05), indicating that the geese may have undertaken short stops en route (of 8 min or less) or, more likely, did not travel in a straight line.

Role of Weather.

In contrast to expectation, the majority of northward-flying geese began climbing during the night or early morning in what were, on the basis of conditions at the Pyramid station and the known prevalence of mountain-valley wind systems in the Himalaya (22, 23), likely to be light headwind (katabatic) conditions, and two geese completed their entire climb before anabatic conditions should have begun. The majority of climbing flights took place before 10:00 h and the four highest climb rates were recorded before 07:00 h (range 1.5–2.2 km·h⁻¹). The last goose to begin its crossing of the Himalaya did not complete its climb in 1 d and landed during the afternoon during what were likely to be peak afternoon tailwinds. This goose did not complete its climb until early the next morning. Another bird stopped climbing before midday and completed the climb during the late afternoon, a period when winds should have been slowing (Fig. 1A). Even when geese did fly in the late morning there were no significant increases in ground speeds or minimum climb rates and, thus, no indication of the use (or presence) of assisting wind conditions. Although headwinds may maximize climb rates relative to the ground, they do so at the expense of horizontal gain. The geese in our study did not appear to trade off horizontal ground speed for vertical gain, traveling at median forward transit speeds during the climbs of 51.9 km·h⁻¹ (n = 18).

Similar results were obtained for geese that we tracked during their southward autumn migration into India (Fig. 1D): the majority of geese took off during the night or early morning, with all but four arriving in India before the transition to anabatic conditions would have been likely to occur. We recorded three of the four greatest rates of descent between 19:00 h and 02:00 h (range 0.9–2.5 km·h⁻¹).

We suggest that these relatively early flight times, for both southerly and northerly migrating individuals, may be suitable because the air is likely to be colder and with lower wind speeds, which could be beneficial in a number of ways. Firstly, cooler air is denser and could, therefore, reduce the costs of flapping flight. For example, at 6,000 m elevation, the maximum observed variation in air temperature at the Pyramid site (19.8 °C) could reduce the altitude effectively experienced by the geese (the density altitude) by 690 m. This would reduce the flight speed required for minimum power consumption by 0.8 m·sec⁻¹, (a 4% energy saving) while maintaining the same lift–drag ratio. Secondly, cooler and denser conditions will increase the partial pressure of oxygen following the ideal gas law (for example, at 6,000 m the change in density altitude by 690 m would increase inspired PO₂ from ~75–81 mm Hg, or 10–10.8 kPa), increasing the diffusion gradient at the alveoli–blood interface and potentially increasing hemoglobin saturation. This increase may be critical in O₂ uptake, as has been shown for mountaineers attempting to reach the summit of Mount Everest (27) without supplementary oxygen. In addition, early flights would avoid the potential heat load of flying at low altitudes in India during the hottest time of the day, whilst cooler nighttime and early morning temperatures could help dissipate metabolically produced heat from the body (maximum daily temperatures in the Khumbu valley were 23.4 °C at 2,660 m, 17.2 °C at 3,560 m, and −10 °C at 5,585 m). In this regard, if bar-headed geese could directly decrease their pulmonary blood temperature at the air/lung interface (4), or indirectly via dropping core body temperature (28), they could improve the O₂ loading and saturation of hemoglobin by leftward shifting the hemoglobin–O₂ equilibrium curve (29). Finally, these calmer periods during the early morning may also be optimal because they provide the birds with an extra measure of flight safety and aerodynamic control, avoiding turbulent and/or stormy weather more prevalent in the afternoon. Indeed, geese flying in the mountains might choose to avoid tailwinds as they would reduce climb angles with respect to the underlying terrain. Calmer conditions may also facilitate the use of formation flight to further reduce energy costs (30).

We make the surprising observation that the majority of climbing flights occurred during the night and early morning and, more often than not, did not overlap with times predicted to provide tailwinds. Lawrence Swan, a naturalist who accompanied Sir Edmund Hillary on an expedition to the Himalayas observed: “On one such cold and still night in early April, I stood beside the Barun glacier… Coming from the south the distant hum became a call. Then, as if from the stars above me, I heard the honking of bar-headed geese” (31). Thus, whether descending or ascending, the bar-headed geese we tracked usually cross the Himalaya when the prevailing wind speeds are likely to be minimal, in a sustained aerobic flight, and flying close to the air speeds that are predicted to maximize power available for the climb. As a consequence, they can maintain maximum safety and control over their flights, while optimizing lift production and oxygen availability. We conclude that bar-headed geese are not reliant on upslope tailwinds to aid flights over the Himalaya and may strategically avoid such conditions while performing these remarkable sustained flights under their own muscular power, even in the relative stillness of the night.