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Charles A. Long, G. P. Zhang, Thomas F. George & Claudine F. Long. (2003)
Physical theory, origin of flight, and a synthesis proposed for birds.
Journal of Theoretical Biology 224: 9-26.
Neither flapping and running to take-off nor gliding from heights can be
disproved as the assured evolutionary origin of self-powered flight observed
in modern vertebrates. Gliding with set wings would utilize available
potential energy from gravity but gain little from flapping. Bipedal
running, important in avian phylogeny, possibly facilitated the evolution of
flight. Based on physical principles, gliding is a better process for the
origin of powered flight than the "ground-up" process, which physically is
not feasible in space or time (considering air resistance, metabolic energy
costs, and mechanical resistance to bipedal running). Proto-avian ancestors
of Archaeopteryx and Microraptor probably flapped their sparsely feathered
limbs synchronously while descending from leaps or heights, with such
"flutter-gliding" presented as a synthesis of the two earlier theories of
flight origin (making use of the available potential energy from gravity,
involving wing thrusts and flapping, coping with air resistance that slows
air speed, but effecting positive fitness value in providing lift and
slowing dangerous falls).
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