Physical theory, origin of flight, and a synthesis proposed for birds.

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).