Birds in Flight
By “Awake!” correspondent in the British Isles
WHEN about to complete their conquest of Mount Everest in the summer of 1953, Sir Edmund Hillary and Tenzing Norgay were astonished to see a large bird flying at 27,000 feet (8,200 meters). Their report of this created considerable interest among ornithologists and bird lovers. It raised such questions as: What variety of bird was involved? If survival required it, could this bird or others fly still higher?
Ornithologists know that birds may fly higher than normal when some instinctive reason demands it. These creatures may do so to clear an obstacle, such as a mountain range, in their path. Or, they may be searching for tastier insects. Birds that have their normal habitat in the lowlands fly at a higher level during migration than at any other time. For instance, migrating pelicans, ducks and cranes have been observed flying at 3,000 to 8,000 feet (900 to 2,400 meters). Lapwings and larks have been sighted at over 6,000 feet (1,800 meters), while plovers and sandpipers have been seen flying at 10,000 to 12,000 feet (3,000 to 3,700 meters). In each case, these heights were sufficient for the birds to clear obstacles in their path. Could they have flown higher?
From observational reports, the answer is, Yes. In the Himalayas some years ago, two types of lowland birds—curlews and godwits—were noted flying at 20,000 feet (6,000 meters).
It has also been observed that high-flying migrating birds may drop down to lower altitudes during bad weather. This suggests that they are not bound into some inflexible flight pattern, but, rather, will instinctively choose to fly under instead of over a storm in order to pass it.
Modes of Flight
All modes of flying may be classified as wing flapping, soaring or gliding. Of these, soaring and gliding are found only in relatively large birds. Low speeds are inherently possible with a soaring or gliding flight, but extra lift is necessary. The bird’s large wings and its weight (giving added momentum) supply this needed lift. The Birds (a volume of the Life Nature Library) gives more details, saying: “Soaring birds, with a large sail surface in proportion to their weight, fall into two very different types: (1) those with broad wings and fanlike tails as exemplified by many of the hawks, eagles and vultures, and (2) those with extremely long but relatively narrow wings, ocean wanderers such as gulls, frigate birds and albatrosses.”
The shape and the size of a bird’s wings in relation to its body weight are also factors that determine its mode of flight. For example, a greater number of wingbeats are needed to keep aloft those birds with wingspreads that are relatively small in proportion to their bodies. Hummingbirds have to use a rapid whirring flight just to keep aloft. Larger birds having a very fast wingbeat, such as partridges and puffins, can also glide, but for only short distances. On the other hand, such large birds as herons and pelicans, with a wingspread that is greater in relation to their body weight, have a slow wingbeat.
Even the hopping birds that we enjoy watching in our gardens often are very seasoned travelers. Blackbirds, thrushes, finches, tits and those tiny bundles of energy, the wrens, travel widely and regularly.
In the autumn vast numbers leave their nesting sites in Scandinavia and eastern Europe and fly south and west toward Britain, to enjoy the milder winters. Then they make their way back in the spring. Radar observations on the east coast of Britain have revealed that these small birds arrive generally at speeds under 30 miles (48 kilometers) per hour and at heights of less than 3,000 feet (900 meters).
If a bird is capable of continuing in the air for a long period, the speed of flight can be a factor influencing its ability to fly high. Sustained flight, in turn, depends on the pattern of flight characteristic of each particular bird variety. This pattern—whether wing flapping or soaring or a variation of both—is determined chiefly by the structure and size of the bird’s wing bones and feathers, as well as by its body weight and the development of its breast muscles.
Fitness Vital for Migration
If a migrating bird must fly at mountain-high altitudes to reach its final destination, it must possess extraordinary stamina. This calls for an incredibly high standard of fitness, a fitness that in most long-distance migrants is dependent on ability to store a great fuel supply in the form of fat. In some bird varieties the doubling of their usual body weight is necessary before such flight is possible.
Effect of Thermal Currents and Different Wind Speeds
Birds may also make use of thermal currents. Such a current begins as a rising column of warm air and is undercut by colder air. Rising quickly, the thermal current is like a large warm-air bubble. As thermal currents rise, birds such as gulls and hawks rise with them. The most spectacular of all European migrants, the white stork, makes use of these thermals at the start of its long journey southward. Setting off from Spain, Holland or the Rhine River region, the bird rides these thermals to a great height before gliding in a southwesterly path to the Strait of Gibraltar and across Africa.
Like other birds, storks soar upward before they start to glide. Ornithologists describe gliding as the simplest form of flight. It is less complicated than flapping or hovering, and saves energy. Doubtless you have observed swallows gliding to and fro, needing only a few strong wing strokes for a long, graceful flight. Many birds use a gliding approach prior to landing, notable among these being wild geese and pelicans flying in formation.
Most efficient of all gliding birds, though, are the albatrosses. The wandering albatrosses, for example, spend nine months of the year on the wing in the southern hemisphere. In their gliding, these birds utilize the difference in speed between the winds at the surface of the ocean and those 50 feet (15 meters) above the surface. Not affected by frictional drag of the ocean waves, the higher air currents are stronger and faster, enabling the albatross to gain speed. Then, when coming to the slower-moving air currents, the bird, traveling at a high speed, is pushed upward.
Since ability to fly high is noted among many large birds, it may seem odd that some varieties of hummingbirds have been observed on the wing at a height of 16,000 feet (4,900 meters). This tiny creature is one of the few birds able to fly backward momentarily. Astonishing, too, is the speed of its wingbeat. It does not seem possible that the ruby-throated hummingbird, found in the United States and Canada, may flap its wings from 50 to 70 times per second. Yes, per second! Contrast this with the wingbeat of the pelican, which needs to flap its wings only 1.3 times a second to keep flying.
Obviously, one would question how the tiny hummingbird could possibly have enough energy to reach 16,000 feet (4,900 meters). But the answer is simple enough. Those hummingbirds seen at this height were in the vicinity of perpetual snow in the Ecuadorian Andes, mountains that range from 8,000 feet (2,400 meters) to peaks over 20,000 feet (6,000 meters). Since these particular birds are known to live high in the Andes, they have a good start in achieving a great height when flying.
‘Dwellers on the Heights’
Hence, it is understandable that other ‘dwellers on the heights’ are observed flying high. Among these is the Andean condor, a giant bird that ranks in size with the California condor and is at home in the highest elevations of the Andes. So, it is not surprising that one of these birds was seen flying with apparent ease in the thin air some 6,700 feet (2,040 meters) above an Andean peak of 12,958 feet (3,950 meters).
Possibly the greatest known recorded height attained by birds in flight was that achieved some years ago by a flock of geese in migration across the lofty Himalayas. At that time scientists were photographing the sun over India when they saw the geese flying in V-formation at the height of Mount Everest, around 29,000 feet (8,800 meters).
What about the high-flying bird seen by Sir Edmund Hillary and his guide? Neither man was able to identify it. It may have been that huge vulturelike bird, the lammergeier. On occasion lammergeiers have been spotted flying at 24,000 to 25,000 feet (7,300 to 7,600 meters) in the Mount Everest area.
Could such birds, the high-flying geese, the condors and perhaps the lammergeier, fly higher if it were necessary? Quite possibly!
[Picture on page 24]
eastern golden plover