Stress, or G-loading, is a measure of acceleration.
As such it allows birds (or pilots!) to sense changes
in the rate of climb, but not the climb rate itself
(which we all know is a velocity, not an acceleration).
I would imagine that birds can use these changes in
acceleration to help find the center of a small thermal
in some cases, but it might be less helpful in larger,
more uniform bands of lift where the ability to integrate
the cumulative acceleration effects over time is more
difficult.

A falconer at the Parowan regionals last week told
us that soaring birds have sensory organs that are
able to measure the pressure differential between the
outside air and inside their hollow bones. I have not
been able to confirm this, but it would seem to make
sense - think of the bones as capacity bottles.

Any bird experts out there?

9B



At 19:48 04 July 2004, Martin Gregorie wrote:
On 04 Jul 2004 18:48:04 GMT, ospam
(Frostowits)
wrote:

Pardon this input from an uninformed intruder to this
subject, but why couldn't
birds simply sense how strong the lift is by the amount
of stress it puts on
their 'airframe'. When I pump iron, I'm all too aware
of the amount of effort
required. Surely birds can do the same.

Some seem to do just that. Kites in particular. I used
to watch them a
lot in India and discovered that you can tell how strong
the lift is
by looking at them - the stronger it is the more dihedral
they use. If
its really strong they just bomb round with a steep
V-form and their
tip feathers closed. Weaker lift gets more care and
attention, less
dihedral and more open tip feathers. When they're really
scratching
their wings are flat or even a little anhedralled and
the tip feathers
are fully spread and up to give tip dihedral. They
initiate a turn
with a big dab of negative in the inner tip and then
control the turn
on tail tilt - the outer tail tip is raised, so you
can tell that
they fly like we do with down force on the tail. Kites
are easy to
read because they often work low altitude lift where
you can see
exactly what they are doing and have big, long tails
that are easy to
observe.

I wondered about how vultures fly but they were so
seldom low enough
to really watch that I couldn't work out very much.
Also, with much
shorter tails than kites its difficult to see whether
they use tail
tilt at all or which way its applied.

The above is about all I know about soaring birds:
I'm no
ornithologist or naturalist. My background is chemistry,
competitive
free flight model flying and, latterly, soaring.

I've heard a number of theories about how birds detect
thermals
including that they hear them. I'd well believe that,
with a nerve on
each feather, they must *really* feel the air and all
its
micro-turbulence. Maybe they can hear it too. However,
that tells
something about how they work 'normal' thermals but
not a lot about
how they can find and work the very weak, smooth lift
you get early
and late in the day. We know that migratory birds have
a excellent
directional sense so why shouldn't a soaring bird have
a built-in
vario too? I'd love to know how it works.

If you haven't read it, find a copy of Philip Wills'
'On Being A
Bird'. There's a chapter about flying with vultures
in South Africa
and how he worked just how vultures operate - altitudes,
spacing, food
finding strategy etc. The whole book is a good read
too.

--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :