Bar-headed geese are true high-flyers. These remarkable birds, with dark stripes across the back of their heads, have been seen flying over Mount Everest, the world's highest mountain. Scientists now find that these geese do not make use of tailwinds that could give them a boost up the mountain and choose instead to rely on their muscle power alone.
The geese spend the winter in India and then, when summer nears, undertake a long journey back to their breeding grounds in Central Asia. To do so, they “must perform the world's steepest migratory flight north over the highest mountain range on earth, the Himalayas,” remarked a team of scientists from a number of countries who tracked some of these birds during their annual migration.
It is estimated that not less than 50,000 bar-headed geese winter in various parts of India, according to S. Balachandran of the Bombay Natural History Society, who was part of the team and is one of the authors of a paper appearing this week in the U.S. Proceedings of the National Academy of Sciences.
Most passes in the Himalayas are at a height of more than five kilometres where the air is thin and oxygen level low. The thinner air means that less lift is generated when the birds flap their wings, thereby increasing the energy costs of flying by around 30 per cent.
However, in the eastern Himalayas, near Mount Everest, winds start to flow up the slope from a southerly direction around 9 a.m. and reach a peak shortly after noon. The winds then reverse during the night and blow southwards. The geese could take advantage of these updrafts and tailwinds by flying in the mid-morning through to early afternoon.
Yet, as scientists watched birds that had been captured and fitted with satellite transmitters in India and Mongolia, that is not what they observed.
The majority of northward flying geese began ascending during the night or early morning when they were likely to face headwinds that could retard their progress. Two geese of the five that were tracked flying north completed their climb before the tailwinds even began. Another, which did not complete the crossing in a single day, landed in the afternoon when tailwinds would have been blowing strongest and completed its climb only early next morning.
Geese that were tracked during their southward autumn migration into India also did not appear to use tailwinds to cross the Himalayas.
The birds could be opting to begin their crossing of the mountain range at night or early in the morning when the air was likely to be colder and winds less strong, the scientists suggest.
Cooler air would be denser and therefore reduce the costs of flapping flight. The greater density would increase the amount of oxygen available to the birds. It would also be easier for them to dissipate the heat generated by their exertion.
Calmer winds could give the geese better control during flight and also help in formation flying that further reduced energy costs.
Scientists have already found that the bar-headed geese are adapted in various ways to flying at high altitudes. Their muscles have a denser network of capillaries that reach oxygen-carrying blood to the cells; their blood is capable of binding and transporting more oxygen; they also have larger lungs for their size and breathe more heavily than other waterfowl.
