Abstract: Statistic analyses of the proportions of the three major bones of the hindlimb (femur, tibiotarsus and tarsometatarsus) from 137 skeletons belonging to 59 families of 18 orders of living birds with different perching habits show that they can be used as a good indicator of their living styles. Measurements of the length of the hindlimb bones indicated that the tibiotarsus is invariably the largest among the three major leg bones. We also found that the femur is shorter than the tarsometatarsus in terrestrial birds while the reverse is true in arboreal birds. We also analyzed the leg bone proportions of 14 species of Mesozoic birds from the Early Cretaceous of China by using ternary diagrams and comparison with 21 specimens of Cenozoic (Paleogene and Neogene) birds，and concluded that the Early Cretaceous enantiornithines are typical arboreal birds, more basal birds from the Mesozoic are mainly arboreal with some adapted for scansorial life, and the ornithurines are characterized by both arboreal and terrestrial adaptations. Our work, combined with the phylogenetical framework of bird evolution, confirms that the arboreal adaptation represents a primitive condition for early birds, which provides further evidence for the arboreal hypothesis of the origin of bird flight. The differentiation of the perching habits of early birds demonstrates that they expanded their ecological niches and food resources during the competitions among themselves and with contemporaneous vertebrates.