Connie and Tweeters,

At first, it might seem logical that birds could fly faster at 4000 or
5000 feet altitude than at sea level, because of the lower atmospheric
density, and hence less drag. However, because Vaux's Swifts (and most
birds) fly by flapping, lower atmospheric density would work against
this by simultaneously reducing the thrust (providing less air to push
against during each wingstroke). The net result, it seems to me, is
that altitude should make no difference to the speed at which birds
fly.

Being a baseball fan, I've heard it claimed that it is a bit easier to
hit a home run in Coors Field, home of the Colorado Rockies (at over
5000 feet altitude in Denver) than in other ballparks of similar
dimensions. I'm not sure if the statistics support this claim.
However, there's a big difference between baseballs and birds--
baseballs don't flap, and the batter can apply equal force regardless
of the altitude!

I suspect that the reason the Vaux's Swifts near Mount Rainier seemed
to be flying faster is just that they were closer to the ground than
you usually see them, and hence the relative motion was greater.
Either that, or there was a Peregrine after them! :-)

Wayne C. Weber
Kamloops, BC
contopus at shaw.ca


----- Original Message -----
From: Constance J. Sidles <csidles at mail.isomedia.com>
To: <tweeters at u.washington.edu>
Sent: Monday, August 05, 2002 4:09 AM
Subject: Mt. Rainier

< snip >

Also in the area was a good-sized flock of Vaux's swifts. What was
unusual about them was their speed. They were flying around much
faster than the swifts do at the Fill. In fact, they were so fast that
they looked like an entirely
different species. I'm guessing that their increased speed was made
possible by the thinner air. Does anyone know if that is true?

< snip >

Connie, Seattle

csidles at mail.isomedia.com