Why do you think the cranes can't climb as fast the ultralight planes? Pilot Joe Duff helps us undertand:
Whooping cranes are soaring birds. They have long narrow wings and low body weight. A five-foot tall Whooping crane only weighs about 16 pounds. A crane uses its large wing span to sail on rising warm air. They fly like hawks or eagles, and don't have to flap their wings very often. Soaring birds are designed to let the air carry them up, which is not nearly as hard work as flapping their wings.
Compare this to large flap-flying birds such a geese and swans. Flap-flying birds have large pectoral muscles and shorter, broad wings. They are generally heavier because they are more muscular.
Most birds can't climb very fast because it's not something they need to do very often. Airplanes, however, need to climb faster because they need to take off from a runway; the faster planes can climb, the shorter runway they need in order to clear obstacles.
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