The bar-headed goose is one of the world's highest-flying birds, having been heard flying across Mount Makalu - the fifth highest mountain on earth at - and apparently seen over Mount Everest - - although this is a second-hand report with no verification. This demanding migration has long puzzled physiologists and naturalists: there must be a good explanation for why the birds fly to the extreme altitudes... particularly since there are passes through the Himalaya at lower altitudes, and which are used by other migrating bird species. In fact, bar-headed geese had for a long time not been directly tracked (using GPS or satellite logging technology) flying higher than , and it is now believed that they do take the high passes through the mountains. The challenging northward migration from lowland India to breed in the summer on the Tibetan Plateau is undertaken in stages, with the flight across the Himalaya (from sea-level) being undertaken non-stop in as little as seven hours. Surprisingly, despite predictable tail winds that blow up the Himalayas (in the same direction of travel as the geese), bar-headed geese spurn these winds, waiting for them to die down overnight, when they then undertake the greatest rates of climbing flight ever recorded for a bird, and sustain these climbs rates for hours on end, according to research published in 2011.The 2011 study found the geese peaking at an altitude of around 6,400 m (21,000 ft). In a 2012 study that tagged 91 geese and tracked their migration routes, it was determined that the geese spent 95% of their time below , choosing to take a longer route through the Himalayas in order to utilize lower-altitude valleys and passes. Only 10 of the tagged geese were ever recorded above this altitude, and only one exceeded , reaching . All but one of these high-altitude flights were recorded at night, which along with the early morning, is the most common time of day for geese migration. The colder denser air during these times may be equivalent to an altitude hundreds of meters lower. It is suspected by the authors of these two studies that tales of the geese flying at are apocryphal. Bar headed geese have been observed flying at 23,000 ft.