Since my last communique on the effects of thin air on bird speed, I
received a very interesting and learned note from Jack Kintner. He reminded
us of the many effects of thinning air as you rise up and attempt to propel
objects through it.

I think that he made many valid points. I am still uncertain whether the
differences in flight speed would be so noticeably faster at 4000 feet that
we would be very impressed by it, but I remain open to this possibility at
all times. At 30,000 feet, the air density differences are certainly
dramatic.

In Kerlinger's "Flight Strategies of Migrating Hawks", he certainly uses air
density as one factor in his formulas for determining air speed, and
coefficients of lift & drag. Unfortunately, (for this thread)
he does not discuss the importance (or lack of importance) of air density.
What seems really effective in changing flight speed is a bird's ability to
change its wing shape, angle of attack and other macro factors that result
in significant changes in speed. These changes work at all air densities,
but may be more effective at some densities than others. This is not dealt
with.
Other key factors which are obvious, include horizontal and vertical wind
speeds.

So it is clearly a complex issue where speed relates both to the
characteristics of the atmosphere as well as the physical ability of the
bird to change its structure and behavior to take advantage of, or correct
for the atmospheric variables. If all else were equal, one could conclude
that if only the air density was the variable, it would affect air speed.
However, one wonders if the birds ever do things so exactly the same that
you could isolate just the one variable and nominate it as the causative
factor in explaining differences observed.

Just out of interest, in relation to the speed of an object, energy provided
and air density, if you started falling at 30,000 feet, would your air speed
slow up as you approached the ground due to more dense air. ? This assumes
that the energy provided ( ie gravity) is not changing.

Thanks for the fun of thinking out this topic.

Rick Howie
Kamloops, BC
rick.howie at shaw.ca