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Re: storing a food source
2009/9/16 jrc <firstname.lastname@example.org>:
> On a long non-stop flight, some birds will burn off up to 50% of their body
> weight. Most of that burn off is fat, followed by muscle mass; in the end
> limited by water required for continuing breakdown of the muscle mass.
I've heard this factoid many times, and I have to say I find it VERY
hard to believe. Brodkorb (1955) tore down a bald eagle carcass and
weighed the components: he found that 25% of the weight was integument
14% feathers, 2% down and 9% skin), 58% was muscle and nervous system,
7% skeleton, 6% digestive tract, 2% respiratory system, 1% heart and
1% kidneys. I imagine that the proportions are broadly similar in
other birds. Assuming that the feathers, down, skeleton, respiratory
system, heart and kidneys are not resorbed during flight, that leaves
73% body mass represented by skin, muscle and digestive tract. For
total body mass to fall by 50%, that 73% would have to become 23%,
i.e. the skin, muscles and digestive tract would have to shrink to
less than one third their starting size.
No doubt migratory birds can start out carrying proportionally much
more fat than the eagle, but losing half of total bodyweight still
stretches at the bounds of credibility. Does anyone have references
for this frequently-cited statistic?
Brodkorb, Pierce (1955). Number of feathers and weight of various
systems in a Bald Eagle. The Wilson Bulletin 67(2):142.