I imagine there are other birding-ausers like myself who have followed the
discussion of wing shapes, moult, etc., with much interest while having no
personal specialist knowledge to contribute.
May I post a query on a related topic - how do gliding birds generate their
forward speed?
Francis Ratcliffe wrote a book "Flying Fox and Drifting Sand" which he says
is "essentially a collection of observations, impressions, and
reminiscences, on the whole more subjective and trivial than scientific and
serious". It is however a fascinating book as is witnessed by the fact
that it has been reprinted several times since its first publication by
Chatto and Windus, London, in 1938. And as the bases of his observations
were his scientific studies as a biologist (Oxford graduate), first of
flying foxes and later of soil erosion, they are well worth reading.
Decades later, he was to became Chief of CSIRO Division of Wildlife
Research.
Here's a relevant sample of his writing. He was inspecting a flying fox
camp in mangroves near Bowen.
"Finding a "white" mangrove, which topped the sea of Rhizophoras by
several feet, I climbed to a precarious perch in a fork of its highest
branch. My presence disturbed the foxes, which began to scramble and flap
away over the top of the foliage. From my vantage-point I tried to
estimate the size of the camp, and was engaged in this hopeless task when
the sound of rushing wind made me look over my shoulder. What I saw was a
wedge-tailed eagle in the act of stooping, and so close on me that I
instinctively threw up my arm to protect my head. Actually it could not
have approached nearer than twenty or thirty feet; but the brute was so
large, and hurtling towards me at such a terrific rate, that my instinct
seemed reasonable enough at the time. The bird must have been attracted by
the movement of the foxes which my presence had disturbed and, incredible
though it may seem, had failed to notice the disturber. Anyway, at the
turning of my head he seemed to see me for the first time, and I was
treated to the unique sight of an eagle putting on the brakes at full
speed. With a startled yelp he seemed to fling the whole expanse of his
wings and body and tail against the line of his dive; and so great was the
pressure against his pinions that the long flight feathers were bent right
backwards. I could see this clearly, and the yellow half-closed claws, and
I could hear the wind fairly whistling through the quills. The manoeuvre
did not seem to take effect at first; as the bird continued in the same
direction, straight at me, for several feet. Then all at once the brake
worked; and with a suddenness which defied the laws of the
rate-of-change-of-momentum the eagle sheered upward and away. The whole
affair was a matter of seconds only; possibly two or three at most passed
from the turning of my head to the exit of the bird from my ken. But what
a moment!"
The braking seems a straight forward enough matter, but the "shearing up
and away"? How? As Ratcliffe says it "defied the laws of the
rate-of-change-of-momentum".
But what caused me to dig out this 60 year old book was the recollection of
the other eagle story he quoted as told to him by Darcy Donkin, the owner
of Meteor Downs, near Springsure. A boundary rider had reported that
eagles were playing up with the lambing ewes and Darcy had a small
aeroplane and a guest, Ronnie Adair, who was also a pilot. Ronnie
suggested an aeroplane eagle hunt. The plane was "an old S. E. bought just
after the War" (WWI). It had a top speed of 75 miles per hour. As
Ratcliffe quotes Donkin's story:
"Ronnie took the controls; I climbed into the front cockpit with a
twelve-gauge gun, and we flew over to where my man had reported that the
eagles were bothering. When we arrived at the place we saw a big fellow
get up on the far side of the paddock. We flew after it and it turned to
meet us. It seemed to want to know what on earth we were. Then it flew
alongside, and it seemed as interested in us as we were in it, for it kept
turning its head and looking at the plane. The bus was flat out, doing its
seventy-five; the eagle kept level, and NEVER MOVED ITS WINGS. Ronnie and
I stared at it hard, and he will bear me out - it never moved a feather.
This went on for two or three minutes. I shouted to Ronnie, 'How the hell
is he doing it?' And he shouted back, 'I'm damned if I know! Why not dong
him anyhow?' So I cocked the gun, and was just about to dong him when the
brute changed its mind and shot ahead. He just shot straight ahead and
left us standing, and still he never batted his wings. Ronnie shouted that
the damn thing was just laughing at us, and he was going home, so we turned
round and came back to the house."
Just a good yarn? Or did it really happen like that? Ratcliffe was
convinced that it did. He writes: "He told it with such enthusiasm and
such vividness of recollection, that I was, and still am, convinced of its
accuracy."
Anyway how do birds achieve apparently effortless forward flight on
motionless wings? Or human-operated gliders for that matter.
(Ratcliffe devotes a full chapter to birds. Well worth a read if you can
locate a copy.)
Syd Curtis at Hawthorne, Qld.
H Syd Curtis
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