EPOW - Ecology Picture of the Week

Each week a different image of our fascinating environment is featured, along with a brief explanation written by a professional ecologist.

11-17 May 2015

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How High Can A Bird Fly?

Bar-headed Goose (Anser indicus), Family Anatidae
Northern India

Credit & Copyright:  Dr. Bruce G. Marcot

Explanation:  This week, the main star is one of the highest-fliers in the world: a Bar-headed Goose ... vying for the altitude title of all birddom.  Often touted to be the highest flier, amazingly Bar-headed Geese can surmount the ridgelines and peaks of the mighty Himalayas.  

Research suggests that Bar-headed Geese follow valleys through their migration path across the Himalayas, staying mostly below 18,000 feet elevation.  But a recent study found that occasional  Bar-headed Geese will reach nearly 21,120 feet (that's an altitude of 4 miles above sea level, or 6,437 meters).  Their physiology provides more efficient red blood cells and greater blood flow with more capillaries, than your average honker.   

Other anecdotal sightings by mountain climbers near the top of Makalu have reported Bar-headed Geese flying far overhead, thus reaching a reported altitude of 29,500 feet (nearly 5.6 miles or 8,991 meters above sea level!).  Tested in wind tunnels, Bar-headed Geese have been found to be able to sustain hyperventilation of super-thin air supplies.  

The mighty Himalayas on a clear day!  The cloud-blown peak on the left
is Mt. Everest, topping at 29,029 feet (8,848 meters), and the
peak on the right is Makalu at 27,825 feet (8,481 meters).
Reportedly, climbers reaching the peak of Makalu have witnessed
Bar-headed Geese flying high overhead.

But also vying for -- and perhaps winning -- the highest-flying bird record is the Ruppell's Griffon Vulture of north and central Africa. 

A Ruppell's Griffon Vulture on a relatively low coarsing flight
in Hell's Gate National Park in the Rift Valley country
of southern Kenya, east Africa. 


Ruppell's Griffon Vultures have been reported to fly as high as 36,000 feet.  In one report from Côte d'Ivoire in east Africa, a Ruppell's Griffon Vulture apparently collided with an aircraft at 37,000 feet (that's 7 miles or about 11,300 meter straight up).  

Let's assume that these statistics are roughly correct, and that this vulture and this goose can reach astounding altitudes during their foraging and migratory flights.  What is a different way of thinking about all this?

Consider that at sea level (elevation zero), we experience what is called one bar of atmospheric pressure.  One bar is equivalent to about 14.7 psi (pounds per square inch).  Laying on the beach at sea level, nearly all of the atmosphere is above you, save for those rare places such as Death Valley that dip below sea level, and for various caves and mines that might also extend below sea level.  

So what is the air pressure in, say, a small single-engine plane flying at an altitude of 5,000 feet AMSL (above mean sea level)?  This is told by the simple formula for calculating air pressure:

where p = the resulting atmospheric pressure measured in bars; 
=  the pressure in bars at height h = 0, so for our purposes here to compare to sea level, p0 = 1; 
= height or altitude measured in kilometers; and 
= a scaling factor which, for h expressed in kilometers, h0 = 7.  
And e is a mathematical constant (called Euler's number) which equals about 2.718.  

The result of all this is that the higher you fly, the lower is the atmospheric pressure ... and it lowers by a negative exponential function, meaning that the pressure drops less rapidly the higher you go.  But it still drops.

If our hypothetical plane is flying at 5,000 feet (= 947 meters or 0.947 km), then p = (1)(e-(0.947/7)) = 0.87 bars.  Remember, at sea level, the atmospheric pressure is (by definition) 1 bar.  So the plane is flying at an altitude where the air has 87% of the pressure as compared to sea level.  Another way to think about this is that the plane is flying above 100 - 87 = 13% of the Earth's atmosphere, by pressure.  


OK, so what about these high avian fliers?

The Bar-headed Goose, flying above Makalu at its record of approximately 29,500 feet or 8,991 meters or 8.991 km, is flying where air pressure is at p = (1)(e-(8.991/7)) = 0.28 bars.  Where it flies, it is above 100 - 28 = 72% of the Earth's atmosphere (again, by pressure)!  

And the Ruppell's Griffon Vulture?  If a plane actually struck one at 11,300 meters (11.3 km) altitude, then the vulture was probably panting for thin oxygen at p = (1)(e-(11.3/7)) = 0.20 bars ... above 80% of the Earth's atmosphere (by pressure)!  


So THIS then is how high a bird can fly!

Think left and think right and think low and think high.
Oh, the thinks you can think up if only you try!
Dr. Seuss

Postscript:  As high as these fliers are winging, they are not yet reaching truly astronomical heights.  Although there is no specific altitude where Earth's atmosphere suddenly drops to zero, and thus where "space" begins, the widely accepted threshold altitude (called the Karman Limit) is at about 62 miles or about 100 km.  The Ruppell's Griffon Vulture might have been seen (bird-struck by a plane) at 11.3 km, so it would still have a long way to go to reach the final frontier.  
     For further perspective, the International Space Station circles the Earth in what is called low Earth orbit, at a range of about 205-255 miles, or 330-410 km.  At that height, there is little risk of the ISS striking a Ruppell's Griffon Vulture...   


Next week's picture:  Khasi Pitcher Plant by the Roadside

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