 |
| A Black Eagle on outstretched wings |
Our recent trip to Dandeli with
the Ogres got me acquainted with a majestic flyer. We were
walking up the Nagzari trail, which leads to a waterfall of the same name, when
a raptor appeared overhead. “Black Eagle!” exclaimed Bijoy. What followed was one of the closest and memorable up-close experiences with
the bird. We strained our necks trying to keep our eyes on the raptor, which
played hide-and-seek as it circled high above the tall trees of the Nagzari Valley.
Our irises followed the Black Eagle (Ictinaetus malayensis) wherever it went, held together as though by an
invisible glue. And then it perched upon a tree with regal poise. We were indeed fortunate to have observed
this bird of prey from so close.
I felt a tingling sensation,
awestruck by the bird's aerial performance against the azure backdrop. I asked Sahastra: “Why do they fly around in circles?”
“Thermals,” he said, and went ahead to explain about the air columns that are formed by the uneven heating of the ground. I wondered: I have been looking at these birds soaring high in the sky for aeons, yet never did it occur to me to delve into the science behind their flight.
“Thermals,” he said, and went ahead to explain about the air columns that are formed by the uneven heating of the ground. I wondered: I have been looking at these birds soaring high in the sky for aeons, yet never did it occur to me to delve into the science behind their flight.
 |
| Watching a raptor, like this Black Eagle, negotiate thermals is an education in itself |
For me large birds circling above
had come to signify death – I had come to assume that the birds circled above
because they had spotted some carrion below. Such an assumption was excusable
during the days when I had not claimed an interest in birding, but not anymore
for I have understood that the soaring flight is yet another classic example of
how our avian friends have adopted a natural phenomenon to their advantage.
This short tête-à-tête with
Sahastra gave me an important insight – what would have been a fleeting glance turned out to be a marveling experience and the reason for it was “interest”.
Reminded me of the story of a lady who threw away an old saucer when an
archaeologist walking by the trash pile jumped up in joy shouting “Ming”. The
interest in the Black Eagle’s flight turned out to be my “Ming” experience,
which helped me understand and appreciate the phenomenon of thermals, which I wish to
share with you.
 |
| Vultures, such as this Himalayan Griffon, offer an excellent demonstration of the phenomenon of soaring, traversing long distances with barely a flap of their enormous wings |
Thermals are rising drafts of air
produced due to the solar heating of the surface of the earth – the air in contact
with the hot surface rises until it loses the heat energy and descends to the
surface of the earth, only to rise again. Thermals are usually observed from late in
the morning (when the sun starts heat up the cold air) until late in the afternoon (when the sun begins to set). The fact that thermals are a localized phenomenon
helps explain why the soaring birds are seen circling – the swirling
updraft of air gives the birds the circular flying pattern. Once done soaring, the birds can move out of the air column and descend by gliding with the cold
downward draft of air.
I chewed on the thought for a
while when a question popped up in my head: Why is it that only certain
species of birds have a soaring flight pattern?
As a bird becomes larger in size, it needs larger wings to produce the
forces required to keep it airborne. For instance, a Griffon vulture weighing
close to 10 kg would need a wingspan of almost 3 metres to sustain flight.
Flapping such enormous wings would require colossal calories of energy (now it is
easy to imagine why ostriches don’t fly) and hence these large birds use
thermals (convection air currents/air columns) to soar.
 |
| Wide wingspans enable large birds of prey like vultures to take advantage of thermals to soar |
Thermals don’t have any
particular direction, so it is possible to observe the birds soaring in either
direction – clockwise or anticlockwise. It depends on factors such as wind direction, the
contours of the surface where the thermals originate, etc. The Coriolis Effect
that guides the directions of hurricanes does not apply here due to the
insignificant air mass of the thermals.
I began writing about my newfound
wisdom on the soaring flight pattern, emphasizing the “hot” air currents
facilitating the ascent of large birds when an email from Sahastra provided
an interlude to my incessant keystrokes.
He shared an experience from
his Himalayan trek with Bijoy where he had observed Griffons and Lammergeiers (Bearded Vultures) rising up against cliff faces on misty/rainy days. He went ahead to explain the concept of obstruction currents - which
are essentially updrafts of air caused by obstructions such as hills, cliffs or
tall buildings. This kind of soaring is called slope-soaring or ridge-soaring. This brought back memories of
my office in Houston (I was on the 19th floor of the 20-storey
building) where I used to see American Black Vultures (Coragyps atratus) flying outside the windows and
occasionally colliding against the panes. I wish I had known about
obstruction currents then -- I would have been able to better appreciate the
presence of these vultures.
The Black Eagle experience at
Dandeli that brought me a learning (better late than never) would
have eluded me had I not heeded to my birder friends’ advice: “When you
spot a bird, don’t just see. Observe!”
Video: Lammergeier soaring
Video: Lammergeier soaring
Lammergeier Descent from Sahastra Rashmi on Vimeo.
Text and info-graphic by Anand Yegnaswami
Photographs by Sandeep Somasekharan
Videos by Sahastrarashmi
Videos by Sahastrarashmi