Re: Mark Chappell's comments, it and previous comments raise a few
questions for me -
Birds during migration can cover 4,000 (or perhaps even more) kilometres -
what mammal/s could do that? I know marathon runners can keep going for
several days so maybe it's just a question of keeping fit. Which birds
have to do all the time otherwise they are dead.
Also presumably there could be a great difference in the metabolic needs of
different bird species and, not being a scientist myself or knowing details
of the studies involved, I wonder how many species have been closely
monitored, or whether even the monitoring methods are valid???
Re: maximum oxygen extraction, birds appear to be able to maintain the very
high levels for very long times and it would appear they also have a great
flexibility to vary oxygen supply at short notice - ie: taking off from the
ground to avoid a predator.
Re: bird song, if you listen to the European Skylark, it keeps a flood of
song going throughout its rise to dizzy heights, but I doubt its song
relates to the rate of wing beat. Often having reached their pinacle, they
keep the same rate of song going for part of the descent when they are
A few items concerning bird lungs....
I've done some research on bird and mammal ventilation (breathing),
although that's not my main area of interest. I suspect that during
flight, there will be synchrony between wingbeat and ventilation cycles --
although not necessarily in a 1:1 ratio. In other words, inhalation might
occur every second, third, or fourth wing stroke (the same thing occurs in
running mammals -- ask any distance runner you know). However, when not
flying the movement of air is generated in the 'usual' way, via muscular
activity not related to wing motion. Birds actually breath fairly slowly
for their mass (relative to mammals) and I very much doubt if breathing
cycles are anywhere near as fast as wingbeat frequency. I once worked on
hummingbirds, which have wingbeat frequencies of perhaps 50 cycles/second
or higher. My resting hummers never breathed faster than maybe 6-8
breaths/second, even at very high metabolic rates (in other words, high
demand for oxygen). I have also studied Adelie penguins; at rest they
breath maybe 6-10 times/minute but when 'flying' underwater they cycle
their flippers several times/second.
Another point alluded to in earlier messages concerned the 'superior'
quality of the avian lung compared to the mammalian lung. It's true that
at extreme altitudes the unidirectional flow in a bird lung seems to work
better than the tidal system in a mammal lung (bar-headed geese routinely
migrate over the top of Mt. Everest, an altitude that will quickly kill
most un-acclimated humans). But there's little evidence that in more
reasonable conditions the bird system is 'superior', in terms of supplying
the oxygen needed for metabolic power production. As an obvious example,
there are flying mammals (bats) which can produce the same power output as
equivalently-sized birds -- and they do this with a typical mammalian lung.
Also, if you look at the maximum power output of birds vs. mammals relative
to the resting metabolic rate, it turns out that the highest performers
are... mammals! The highest power output from birds is maybe 15-20 times
greater than resting metabolism. However, in certain athletic mammals
(racehorses, antelope, kangaroos), maximum power output is 40 times greater
than resting metabolism. Good human athletes can accomplish a 15-20 X
increase above resting metabolism.
Finally, it was suggested in an earlier message that the unidirectional
bird lung removed "most" of the oxygen in respired air. This is a little
bit of an exaggeration. We physiologists use the term "oxygen extraction"
for the ratio of [oxygen in expired air/oxygen in inspired air]. A
typical oxygen extraction for a resting bird is about the same as for a
typical resting mammal -- 20-30% or so (this means that inspired air
contains 21% oxygen and expired air contains 15-17% oxygen). The maximum
oxygen extraction measured for birds is about 60-70% -- very high, but this
occurs only in a few species and only under special conditions (like when
it's very cold). And some mammals can extract almost as much oxygen.
So although from our human engineering perspective the bird lung seems to
be a better 'design' than the mammalian lung, under most circumstances the
ability of birds to supply oxygen for metabolic power production doesn't
appear to be 'superior' to mammals.
Hope this is of interest and doesn't contain too much nerdy scientific
jargon.... Mark Chappell
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