How Fast and High Do Birds Fly?
birds follow the facetious advice often given to pilots --
"fly low and slow." Most cruise speeds are in the
20-to-30-mph range, with an eider duck having the fastest
accurately clocked air speed of about 47 mph. During a
chase, however, speeds increase; ducks, for example, can fly
60 mph or even faster, and it has been reported that a
Peregrine Falcon can stoop at speeds of 200 mph (100 mph may
be nearer the norm). Interestingly, there is little
relationship between the size of a bird and how fast it
flies. Both hummingbirds and geese can reach roughly the
same maximum speeds.
There is, of course, a considerable difference between the speed at which a bird can fly and the speed at which it normally does fly. When the bird is "around home" one might expect it to do one of two things, minimize its energy use per unit time, that is, minimize its metabolic rate, or m e the distance it travels per unit of energy expended. A vulture loitering in the sky in search of prey might, like the pilot of an observation aircraft, maximize endurance; a seabird traveling to distant foraging grounds might, like a Concorde encountering headwinds on a transoceanic flight, maximize range. Staying up longest does not necessarily mean going farthest. A bird might be able to stay aloft 6 hours at 15 mph (maximum endurance, covering 90 miles) or 5 hours at 20 mph (maximum range, covering 100 miles). Birds can also choose to maximize speed, as when being chased by a predator or racing to defend a territory. Or they can choose some compromise between speed and range.
In order to determine what birds normally do, Gary Schnell and Jenna Hellack of the University of Oklahoma used Doppler radar, a device similar to that used by police to catch speeders, to measure the ground speeds of a dozen species of seabirds (gulls, terns, and a skimmer) near their colony. They also measured wind speeds with an anemometer, and used those measurements to estimate the airspeeds of the birds. (The wind speeds were generally measured closer to the ground than the birds were, which led to some errors of estimation, since friction with the surface slows air movements near the ground.)
Airspeeds were found to be mostly in the 10-to-40-mph range. The power requirements of each bird at each speed could be calculated, and that information was used to establish that the birds were generally compromising between maximizing their range and minimizing their metabolic rates with more emphasis on the former. Airspeeds varied a great deal, but near the minimum metabolic rate rather large changes in airspeed did not require dramatic rises in energy consumption. For example, a gull whose most efficient loiter airspeed was 22 mph could fly at anything between 15 and 28 mph without increasing its metabolic rate more than 15 percent.
Most birds fly below 500 feet except during migration. There is no reason to expend the energy to go higher -- and there may be dangers, such as exposure to higher winds or to the sharp vision of hawks. When migrating, however, birds often do climb to relatively great heights, possibly to avoid dehydration in the warmer air near the ground. Migrating birds in the Caribbean are mostly observed around 10,000 feet, although some are found half and some twice that high. Generally long-distance migrants seem to start out at about 5,000 feet and then progressively climb to around 20,000 feet. Just like jet aircraft, the optimum cruise altitude of migrants increases as their "fuel" is used up and their weight declines. Vultures sometimes rise over 10,000 feet in order to scan larger areas for food (and to watch the behavior of distant vultures for clues to the location of a feast). Perhaps the most impressive altitude record is that of a flock of Whooper Swans which was seen on radar arriving over Northern Ireland on migration and was visually identified by an airline pilot at 29,000 feet. Birds can fly at altitudes that would be impossible for bats, since bird lungs can extract a larger fraction of oxygen from the air than can mammal lungs.
SEE: Wing Shapes and Flight; Soaring; Flying in Vee Formation; Adaptations for Flight.
Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.