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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

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

Guy Leahy