‘On a wild goose chase: birds over the roof of the world’, a talk by Dr Lucy Hawkes
Our increasing ability to track animals is revealing a lot of new and sometimes amazing information. Bar-headed geese have been tracked flying at 7,290m and have been reported flying over the peak of Everest (8,848m). Humans climbing the mountain wear oxygen masks and move very slowly. The geese fly at a normal speed, flapping their wings. Snarge left on a plane after a bird strike revealed that a collision with a vulture occurred at 11,270m. Humans show altitude symptoms at 4,000m, are hallucinating at 7,000m, and cannot survive for more than a short time at over 8,000m.
Flight has enabled birds to colonise areas which are inaccessible to other animals, but consumes a lot of oxygen. It is made possible by their unique respiratory system. Human lungs, which contain many tiny air sacs, act like bellows taking in oxygen when they expand and pushing out carbon dioxide when they contract. Birds on the other hand have relatively small static lungs and nine huge air sacs. The air sacs expand and contract providing a continuing unidirectional flow of oxygen-rich air to the lungs where gas exchange occurs. This much more efficient system allows them to survive at altitudes where we could not.
In a 1968 experiment where air was pumped out of a jar to simulate high altitude conditions, a mouse was comatose at 6,000m whereas a house sparrow was still alert. Another experiment showed that a Canada goose was uncomfortable and unable to stand at 10,668m while a bar-headed goose became unsteady and had difficulty holding its head up at 12,190m. Bar-heads have special haemoglobin with a higher affinity for oxygen than closely related lowland geese and they also have greater flight muscle capillarity. Experiments on a treadmill and in a wind tunnel showed that they lost less oxygen than expected. They have evolved to fly at great heights and there is still much to learn about them.
Lucy has been fortunate enough to study wild bar-headed geese in Mongolia over several seasons. It is a hard country weather-wise but the people are very hospitable and yurts are comfortable. Some 6,000 geese spend three weeks by a large lake while they are flightless but they can run fast, swim fast and dive and initially proved almost impossible to catch. However, using inflatable kayaks and working slowly and silently the team was eventually successful. Tracking devices were attached to some birds. The device recorded the bird’s location once an hour for a whole year showing the route that they took to and from southern India where they winter on salt marshes. None flew over the summit of Everest but a height of 7,290m was recorded. On the return flight they have a steep climb as they reach the Himalayas. They usually started the ascent at about midnight (as climbers do). In the morning the wind is blowing south so they were flying into it but it is cold so the air is dense resulting in more oxygen being available. This makes it less costly than it would be in the afternoon even though they would have the wind behind them. They used a flapping flight throughout.
Heart rate loggers were implanted into a few geese in Mongolia and these showed that for most of the time the heart rate was 300 times a minute as compared to 500 times a minute on the treadmill. The birds beat their wings four times per second for most of the journey even when going down over the mountains.
The team wanted to see whether the geese undertook any training prior to their migration south but there did not appear to be any difference in their behaviour during the month prior to departure. They also wanted to find out if they got cold. Exercise such as flight generates heat and the body temperature was found to be about 40 degrees C whenever they were flying. When they woke up in the morning their body temperature went up and when they went to roost in the evening it went down. On migration they were up and flying all night using energy, but in colder less windy conditions than by day. There was considerable variation between individuals but each retained a fairly constant body temperature.
We thank Lucy for a most interesting talk which covered topics that were probably new to most of us. It was really fascinating to learn how the bar-headed geese and other birds are adapted to enable them to fly at heights where we could not survive.
Helen Baker