Kvist's team trained four red knots, each of a different body mass, to fly in a wind tunnel for between six and ten hours and measured how much power was required for each bird to fly.
The researchers found that heavier birds did not expend as much energy as expected to carry the extra weight.
Apparently the heavier birds use their muscles more efficiently, says bird conservation specialist Trevor Lloyd-Evans of the Manomet Center for Conservation Sciences in Massachusetts. Flying with a greater fat, or fuel, load on long migrations is worth the energy required to carry the extra weight, added Lloyd-Evans.
A third team of researchers led by Henri Weimerskirch, of the Centre National de la Recherche Scientifique in Villers en Bois, France, measured the energy saved by flying in formation.
Weimerskirch's team trained great white pelicans to fly after a motorboat and an ultralight plane while they measured the animals' heart rates during flight and filmed the birds to determine the frequency of wing beats.
The researchers found that pelicans flying in formation had a heart rate that was between 11 and 14 percent lower than that of pelicans flying solo. They also found that pelicans flying in formation beat their wings less frequently than solo birds and were able to spend more time gliding.
Migrating birds take note: fatten up, fly in formation, and pay attention to the Earth's changing magnetic fields.
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