Flight Styles: Thermalling

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Birds that use thermal uplift to conserve energy as they gain altitude typically have long broad wings and a comparatively short broad tails. This Black Vulture was photographed in Montgomery County, Texas. (Photo by Scott Carpenter).

In our first investigation of avian flight strategies we explored dynamic soaring and how some birds are able to take advantage of wind gradients (differences in the speed of two air masses) to cover great distances with nary a wingbeat. 

In this piece we will explore how soaring birds exploit the temperature differences in neighboring air masses. Vultures, hawks, pelicans and cranes are among the species one most often sees gliding in tight circles and using thermal uplift to gain altitude with very little wing flapping.

Rising columns of warmer air (thermals) are created by the solar heating of the Earth's surface. Areas with the most exposure to the sun warm more rapidly than areas that are shaded, thus the air masses above exposed areas also warm more rapidly. 

Since warm air masses are less dense, they rise while the dense cooler air masses descend, in essence adding a push to the already rising columns of warmer air. When a soaring bird encounters these rising air masses it starts circling in an effort to remain within narrow column of rising air, which carries it upward. Other soaring birds or flockmates recognize that they are riding a thermal and join in to form a "kettle." It is not unusual to see multiple species of vultures (Black and Turkey Vultures in North America) and hawks together in a single kettle. When birding in south Texas in Winter one of the best ways to find Zone-tailed Hawks is to look for kettling vultures as they make their mid-morning departures from roost sites. Roost sites are often located along rivers or other bodies of water. The land heats up quickly while the water absorbs most of the insolation rather than reflecting heat back into the atmosphere, presumably creating a significant difference in the temperatures of neighboring air masses (see illustration below).  

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This graphic illustrates the interaction between warm columns of rising air (thermals) and descending masses of cooler air. (Image source: http://www.aerospaceweb.org)

If one watches long enough, kettling birds will reach the apex of the rising column and start peeling off the top of the thermal in a long gradual straight-line descent until they find another thermal, where the process is repeated. On 16 September 1993, near Danville, Hendricks County, Indiana, I watched as a southbound group of 103 Broad-winged Hawks followed the pattern shown in the graphic above. The birds streamed single-file into a thermal and formed a kettle almost directly overhead. The hawks were perhaps 200 feet up when they entered the thermal. They then spiraled upward several hundred feet before peeling off the top of the updraft single-file. They glided south about a half mile or so, caught another thermal and kettled up again. It was amazing to watch. I envisioned this process being repeated over and over again all the way to their South American wintering grounds. On many other occasions I've observed transient flocks of American White Pelicans and migrant Sandhill Cranes move along from thermal to thermal.

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The airborne grace of soaring American White Pelicans belies their bulk, which may approach 20 pounds. These pelicans were photographed over Othello, Adams County, Washington on 7 August 2010 (Photo by Steven Mlodinow). 

Clearly, there is a significant conservation of energy that comes from utilizing this strategy.  According to Knopf and Evans (2004), the weight of most American White Pelicans likely ranges from 5.4 to 9.0 kg (12-20 lbs). Getting that amount of mass airborne and keeping it there would require a lot more energy in the absence of thermal updrafts. Like vultures and many hawks, pelicans have comparatively long, broad wings and relatively short, broad tails. They do most of their commuting and longer flights between mid-morning and mid-afternoon when the most direct sun angles increase surface heating. It is always impressive to watch as these massive birds rise almost out of sight with almost no expenditure of effort.

Literature Cited:

Knopf, Fritz L. and Roger M. Evans. 2004. American White Pelican (Pelecanus erythrorhynchos), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online; http://bna.birds.cornell.edu/bna/species/057doi:10.2173/bna

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