birding-aus Re: bird flight

Subject: birding-aus Re: bird flight
From: Mark Chappell <>
Date: Wed, 5 May 1999 15:37:42 +1000
*** I'll give these a try..... hope I don't sound too much like a stuffy
Uni prof (which I am)....

>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.
**** There are some bats that migrate for fairly long distances but as far
as I know, none go as far as many migrant birds, expecially without
stopping.  For example, golden plovers and several other shorebirds go from
Alaska to Hawaii nonstop (about 4000-4500 km); ruby-throated hummingbirds
-- which weigh about 3.5 grams -- cross the Gulf of Mexico non-stop
(900-1000 km); blackpoll warblers (maybe 25 g?) fly nonstop from New
England in the US to northern South America (perhaps 5500 km).  However,
this kind of endurance capacity has more to do with flight efficiency (how
much it 'costs' to transport a certain mass a certain distance) and fuel
management than power output.  Bats are specialized for slow speed and high
manuverability while many migrant birds are specialized for fast, long
distance flight.  [Also, many bats have evolved an ability to hibernate to
avoid winter food shortages, instead of migrating.]  The big questions for
us physiology types are how birds manage to carry enough fuel and how they
can maintain water balance during migration (when you fly you have to
breath a lot, and you lose water when you breath).  There's a bit of debate
as to what IS the limiting factor -- fuel or water.  I've seen a few small
passerines mist-netted just before a long migration flight and they are
absolutely 'squishy' with stored fat.
>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???
*** Only a small fraction of the world's bird species have been studied,
but in terms of flapping (powered) flight, there doesn't seem to be much
variation in power requirements aside from that due to mass differences.
Apparently it requires a certain power output to fly regardless of what you
are, so that large insects, small birds, and small bats (in the 3-10 gram
range) all have about the same metabolic rates during flight.  The methods
used to measure flight metabolism are usually gas exchange (oxygen
consumption or CO2 production) which are very reliable (the trick is
training your bird to fly in a wind tunnel).  However, there's quite a bit
of uncertainty about the MAXIMUM possible power output of birds -- just
because you measure the cost of flying doesn't mean that you've measured
the highest possible power output of that bird.  The consensus is that
hovering is the most expensive form of flight (aerodynamic theory supports
this view).  There is a research group in Texas that uses some ingenious
methods to push hummingbirds to their limits (like asking them to fly with
weights, or in a helium-oxygen atmosphere that is much less dense than
air).  Their results indicate that 'normal' hovering IS about as much power
output as a bird is capable of.
       Of course, gliding flight is much cheaper than flapping flight.  In
fact, probably the lowest daily energy expenditure (DEE; this is the
metabolic power output averaged over an entire day, measured with isotope
turnover) known for an active vertebrate (i.e., one that's out there moving
around and doing stuff, not just resting) is from wandering albatross.  If
I remember correctly, this was less than 2 times resting metabolism.  Most
species of birds and mammals have DEEs of 3-7 times resting metabolism (and
there's no real difference here between birds and mammals).

>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.
**** Actually, power production based on oxygen consumption -- or "aerobic
energy metabolism" in geek-speak -- is only relevant to sustained activity
(i.e., more than a minute or so).  Burst activity (like sprinting) relies
on anaerobic mechanisms which supply A LOT of power, but only briefly
(that's why you can run 100 meters much faster than you can run a
marathon).  Different muscle fibers are specialized for burst or
sustainable activity.  [--> a slight digression: The burst fibers are
'white' muscle -- like a chicken breast -- and the endurance-specialized
aerobic fibers are 'red' muscle -- like a chicken leg.  This dichotomy is
true of all vertebrates.  You can REALLY see it nicely in a cross-section
of a swordfish or shark -- most of the fish's muscle is white fibers used
for burst swimming and the 'routine' cruising is handled by the little gob
of red muscle in the middle of the fish.]  A bird taking off to avoid a
predator is probably using every muscle fiber it's got, but it won't be
able to sustain that high activity for very long.  As to endurance, birds
are pretty good at this but they tend to fly at a speed which minimizes the
'cost of transport' (fuel used per unit mass per unit distance), and most
measures of flight metabolism in steady-state flapping flight are around
7-12 times resting metabolism (the higher numbers I mentioned previously
are for hovering).  Flying bats apparently have somewhat higher average
power output, and certainly many running mammals (like horses and humans)
can do the same for long periods.
>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
**** We (or at least I) don't know very much about how song is produced but
I seem to recall that the syrinx (the 'voice box' equivalent in birds) is
so versatile that it works when air is moving in either direction.  Song
production is one area where the complex air sac system may work better
than a simple tidal system.
-- Mark C

       Mark Chappell, Biology Department, UC Riverside
              until Aug '99:  C/O Dr. Bill Buttemer
       Dept. Biological Sciences, University of Wollongong
       Wollongong NSW 2522 AUSTRALIA

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