In this paper, two traditional hypotheses for the origin of birds' flight, namely the "arboreal" hypothesis and the "cursorial" hypothesis are intruduced. According to the findings of feathered dinosaurs and primitive avialian birds in western Liaoning, it is more likely that the ancestors of birds used a combination of taking off from the ground and taking advantage of accessible heights (such as hills, cliffs, large boulders, or fallen trees). It seems not necessary that the ancestors of birds must climb trees to assist flight.