Christmas Bells

Christmas Bells
Christmas Bells - Blandfordia nobilis

Sunday, July 16, 2006

The Nature of Eagle Flight

A moderate, but steady wind was blowing from the south-west at my place, today. As I am near the top of a ridge, with a valley facing south-west, below me, this means that the wind was running straight up my valley, creating wonderful "uplift", just above my house. This is a perfect situation for the Little Eagle (Hieraaetus morphnoides). This bird did a number of "passes" up and down the valley, today.


The bird would circle around to gain height, as it came back up the valley (photo 1).

Clearly it was hunting, for it was not just "making lazy circles in the sky" (to quote from the song in "Oklahoma"). Once the bird has got high enough up the valley, to get the desired "uplift" from the ridge below, ti would "freeze" (hover). But although it was perfectly stationery, it was working very hard, adjusting its wings and tail constantly, in order to keep its head absolutely still. On several occasions, it would hold the one position (relative to the ground) for a minute or more, at a time, before moving off a little further, then holding position again - but it was working hard to achieve this "fixed" position.

See the various photos below, to observe the different wing and tail "settings" the bird was using.


Medium-large birds of prey, such as this Little Eagle rely on using "uplifts" to achieve their hovering effect. Hence today's demonstration, just over the back of my deck.

This shows perfect balance of uplift against gravity, with wingtip control.

Having circled back up from the bottom of the valley, when it found a position high enough in the valley to get the "uplift" again, it then "froze" in the sky, looking for prey. Rabbits and lizards are its main food, but this species has been recorded taking birds, and insects. Apparently it has a preference for young rabbits (which would suit its size).


On several occasions it had to spread its wings and tail to the fullest possible extent, in order to "hold position" - but it was not flapping. It was still stationary in the sky.

Presumably this is in response to a momentary drop in the wind strength, requiring the bird to maximimise its wind resistance (for just a second or two), to hold its position, before the wind picked up again.

Note the "concentration" evident from the positon of its head. This bird was actively hunting. It is scanning the ground below, for a careless rabbit, or possibly a bush rat.


The bird was constantly adjusting the settings of its wings and its tail, to keep its head stationary. It is not moving across the sky - it was stationary, relative to the ground.


Then it would move to a new position, a little further away, across the valley, and repeat the process.

Having moved perhaps as little as 50 metres, it would resume the previous hovering stance, with just wingtip control being used to keep its position in the sky.

What a wonderful demonstration of flight control. My thanks to the Little Eagle, and the wind conditions which made this display possible.

It is great to be alive - to appreciate these special Nature of Robertson moments.

In the great Aussie vernacular: "Ya wouldn't be dead for quids".

No comments: