Michael Norris wrote:
> Actually I got the idea of Mistletoebirds being recent arrivals from a
> famous ornithologist, including his suggestion (as I recall it) that
> they did not carry enough calories to cross the Bass Strait.
If he/she said that they were incorrect. A little fat does a long way.
Assuming their efficiency is not too different to some of the long
distance migrants that have been studied, one gram of fat would take a
Mistletoebird several hundred kilometres and hence across Bass Strait.
Mistletoebirds are in the region of 10 grams so their fat reserves should
be able to reach several grams.
Fat can fuel humans over large distances too. I reckon burning 1%
of my bodyweight in fat will fuel me for about 90km running. I'm very
lean but I still have enough fat to run about 500km.
Birds have some major advantages though. Some long distance migrants bulk
up to 50% body fat. If I did this my running efficiency would plummet -
the energy I consumed per kilometre would almost double. Large amounts
of body fat don't have similar impact on birds. Increased body fat has
even been found to improve flight effciency in some cases.
Humans also can't metabolise fat nearly fast enough to sustain high
exercise intensities. If I exercise at high intensity less than half
of the energy will be coming from fat reserves and these other reserves
will become depleted after about two hours exercise unless I refuel
during the exercise. Some birds can metabolise fat fast enough to
fuel high-exercise intensity. This allows long-hop migrants to fly
continuously for many hours without refuelling.
Andrew
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