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Migration of Birds

Migratory Flight Altitude

While factors regulating the heights at which birds migrate are not clear, there are many obvious reasons why flying at higher altitudes may be advantageous. High-altitude flight may be used to locate familiar landmarks, fly over fog or clouds, surmount physical barriers, gain advantage of a following wind, or maintain a better thermoregulatory balance.

In general, estimates of bird heights based on direct observation are quite unreliable except under special conditions. A Eurasian Sparrowhawk could be distinguished at 800 feet but disappeared from site at 2,800 feet. A Rook (a European member of the crow family) could be recognized at 1,000 feet but disappeared from sight at 3,300 feet. An interesting experiment with an inflated model of a vulture painted black with a wing span of 7 feet 10 inches illustrated similar limitations. When released from an airplane at 4,700 feet, it was barely visible and invisible without binoculars at 5,800 feet. At 7,000 feet it was not picked up even when 12 power binoculars were used. Radar studies have demonstrated more accurately than human vision that 95 percent of the migratory movements occur at less than 10,000 feet, the bulk of the movements occurring under 3,000 feet.

Yet birds do fly at higher altitudes. Bird flight at 20,000 feet, where less than half the oxygen is present than at sea level, is impressive if only because the work is achieved by living muscle tissue. A Himalayan mountain climber at 16,000 feet was rather amazed when a flock of geese flew northward about two miles over his head honking as they went. At 20,000 feet a man has a hard time talking while running, but those geese were probably flying at 27,000 feet and even calling while they traveled at this tremendous height. Numerous other observations have come from the Himalayas. Observers at 14,000 feet recorded storks and cranes flying so high that they could be seen only through field glasses. In the same area large vultures were seen soaring at 25,000 feet and an eagle carcass was found at 26,000 feet. The expedition to Mt. Everest in 1952 found skeletons of a Northern Pintail and a Black-tailed Godwit at 16,400 feet on Khumbu Glacier. Bar-headed Geese have been observed flying over the highest peaks (29,000+ feet) even though a 10,000-foot pass was nearby. Probably at least 30 species regularly cross these high passes. Other accurate records on altitude of migratory flights are scanty, although altimeter observations from airplanes and radar are becoming more frequent in the literature. For example, a Mallard was struck by a commercial airliner at 21,000 feet over the Nevada desert. Radar observations have revealed that birds on long-distance flights fly at higher altitudes than short-distance migrants. It has been hypothesized that advantageous tail winds of greater velocity are found higher up and that the cooler air minimizes the demand for evaporative water loss to regulate body temperature under the exertion of flight. Radar studies also have shown that nocturnal migrants fly at different altitudes at different times during the night. Birds generally take off shortly after sundown and rapidly gain maximum altitude. This peak is maintained until around midnight, then the travelers gradually descend until daylight. Thus, there is considerable variation, but for most small birds the favored altitude appears to be between 500 and 1,000 feet. Some nocturnal migrants (probably shorebirds) fly over the ocean at 15,000 or even 20,000 feet. Nocturnal migrants also fly slightly higher than diurnal migrants. Observations made from lighthouses and other vantage points indicate that certain migrants commonly travel at altitudes of very few feet to a few hundred feet above sea or land. Sandpipers, Red-necked Phalaropes, and various sea ducks have been seen flying so low they were visible only as they topped a wave. Observers stationed at lighthouses and lightships off the English coast have similarly recorded the passage of landbirds flying just above the surface of the water and rarely rising above 200 feet over the waves.

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