Altitude effects on hummingbird flight?

[don ohmes <d_ohmes@yahoo.com>]

---- Original Message ----
From: Guy Leahy <xrciseguy@sbcglobal.net>
To: dinosaur@usc.edu
Cc: jrccea@bellsouth.net
Sent: Thursday, December 21, 2006 1:24:09 AM
Subject: Altitude effects on hummingbird flight?

The January issue of National Geographic has a
pictorial article on hummingbirds.  On page 126 the
text states regarding one species(the Great
Sapphirewing) that:

"The wings of this high-altitude species are unique
for their iridescent blue feathers and their size:
Thin Andean air requires larger wings and fuller
strokes."

---------
I think the way it works is this; wing stroke amplitude increases as
density decreases, but decreases as wing area increases (weight held
equal).  Note that as w/s amp approaches 180 degrees, flight failure
occurs. In hovering flight, wingbeat frequency doesn't seem to vary
much (Chai and Dudley, J Exp Biol 199, 1996).

A question... why would this be so?  Does this mean
that above a certain altitude, hummingbird flight
mechanics might not work?  

---------- Absolutely.

The article does note on
page 123 the largest species of hummingbird is known
to occur at altitudes above 15,700 feet in the Andes,
so it would seem that any altitude constraints on
hummingbird flight mechanics are small.

The article also (on page 122) notes the discovery of
30-million-year old hummingbird-like fossils from
Germany, suggesting that either (a) hummingbirds were
once distributed more widely than the Americas, or (b)
the German fossils represent an extinct group of birds
which evolved convergent flight mechanics to those of
true hummingbirds.

----------
Hummingbirds are the largest observed species using hovering, a
size-constrained locomotion; if some "un-related" group of similar size
evolved hovering, it seems to me that a high degree of convergence
would be expected. In either case, I wonder what could have knocked
them out? Given that hummers occur in Alaska, it astonishes me that
they never got a foothold in Asia. Anybody know got a clue?

----------- Don

Guy Leahy