The synchronization of feet is especially suitable for arboreal birds. When they move in the tree, their feet can grasp the branches at the same time, which is conducive to maintaining the stability of their bodies. Birds that have developed this way of walking can only jump when the ground moves.

In addition, compare the difference between sparrows and pigeons when they take off. If a pigeon wants to fly, it must put its feet together first. Sparrows take off without fitting their feet, so they take off faster than pigeons, which buys time for escape.

This is because sparrows have short limbs, consisting of thighs, leg bones, tarsal bones and toes. The muscles of the whole hind limb are distributed in the thigh and gaskin, and the other parts are tendons. These tendons run through the toes, which can control the bending of the toes and enable sparrows to grasp the branches and live safely.

However, there is no joint socket between Gaskin tarsal bone and tarsal phalanx of sparrow's hind leg, so the joint between tibia and tarsal bone can't bend, which leads to the sparrow can't walk on the flat ground and can only jump quickly and frequently.