An international team of researchers led by the Bangor University in North Welsh found new interesting facts about the migration strategies employed by bar-headed geese. The new findings reveal that, during migration, these birds do not continuously fly over the breath-taking Himalayan peaks, as previously believed, but they choose to fly through Himalayan passes whenever they found one.
The study also challenges previous beliefs that bar-headed geese use tail winds blowing up the Himalayas to push them from behind and boost their speed. Moreover, researchers found that these strange birds use a unique roller coaster-like flight pattern whenever they fly past the mountains to save oxygen and energy needed for their continuous flapping.
Scientists were able to learn all these exciting new facts about bar-headed geese by implanting GPS trackers in two of them during their annual winter migration.
Bar-headed geese (Anser Indicus) are the only known species of birds to fly at extreme altitudes and not get high altitude sickness or die in the process. Past studies have shown that their bodies are also adapted to a low intake of oxygen when flying high over the Himalayan range. They have 25 percent larger lungs than other geese, while their heart and muscles are highly vascularized, i.e. they have more blood vessels. So, their blood can store more oxygen.
Bar-headed geese spend the summer in Mongolia, Tibet, Russia and China and winter in the warmer regions of northern India. They perform a couple of annual migrations – one from Central Asia to India (during fall) and another one back home (in late May). During summer, the female builds a nest by digging a hole in the ground which gets lined with the feathers from the bird’s own breast. Usually, bar-headed geese lay three to eight eggs which hatch a month later.
A bar-headed goose can reach heights of 4 miles (6,300 meters) when flying above the Himalayas or the Tibetan Plateau towards India. The northbound geese start their trip from sea level and must get past snow-capped mountains, while southbound geese have a head-start because they start their journey from the Tibetan Plateau and have much less climbing to perform. It usually takes 4.5 hours or less for a southbound goose to finish its trip, while northbounds need 7 to 8 hours to get on the other side of the mountain range.
“It’s pretty special that a bird as big as a goose — a 2 or 3 kilo bird — can go through such a complex route, through 500 kilometers of some of the highest land on earth,”
Charles Bishop, lead author of the study and researcher from Bangor University, said about northbound bar-headed geese.
Mr. Bishop said that the initial goal of the study was to reveal how these birds succeeded in reaching the extreme heights and continuously fly for hours on end. His team of researchers planned to discover what physiological mechanism allowed the birds to “perform such an athletic feat” at extreme altitudes where air gets so thin that they need to do some extra flapping to generate forces from the thin air.
The GPS tracking devices implanted into the geese could monitor their migration path, altitude, and air pressure, as well as the birds’ accelerations and heart rate. The devices were recovered when the geese finished their migration toward their breeding grounds.
Scientists learned that the birds did lots of ups and downs during their journey, rather than flying steadily at high altitudes. They seem to have a roller coaster-like flight pattern by flying higher to get past mountain peaks and suddenly swoop down into lower regions.
Mr. Bishop confessed he was surprised by the findings since he and his team were convinced that the birds hadve a steady high altitude flight path while crossing the Himalayas. However, flying at 6,000 meters (nearly 4 miles) is very exhausting even for bar-headed geese.
Researchers explained that at the sea level the atmospheric pressure is at 100 percent, while the amount of oxygen is at 21 percent. At an extreme altitude of more than 6,000 meters both air pressure and oxygen level drop to a third. And it is not very easy breathing and continuously flopping in a 7 percent level of oxygen environment.
The study also revealed that bar-headed geese do not make use of tally winds to propel them from behind. Instead these birds choose to fly early in the morning or during nighttime when the air is colder, but also more dense and richer in oxygen.
Additionally, they used strong updrafts of air to boost their climbing rates by two to three times and save energy.
Andrew Farnsworth, bird expert who was not involved in the study, said that the new research was exciting for both migration biologists and the general public interested in extreme animal behavior.
Study: Bar-Headed Geese Use Roller Coaster-Style Flight Strategy.