Nature has given many birds distinctive toes that help in maintaining upright pose even when asleep on a tree branch where as a man would have to strive hard to keep a firm grip on the branch with his arms around it (and remain wide awake too!). The rule regarding the grip applicable to the birds is diametrically opposite to the rule applicable to the human beings. Having perched on a branch the bird has to make an effort to loosen its grip and until it has made the necessary effort it can not let go of the branch. The muscles of a perching bird’s toes – three on the front and one on the back – are like automatic hinges. As the bird alights on a branch the hinges are pressed by its weight. The pressure locks the toes around the branch effortlessly. The effort will be required to relax the grip when the bird wants to move on the branch or wants to fly away.
Nature has made this anatomical arrangement only for those birds that perch on the trees (as well as on telephone/electric lines). For example, the birds that spend all the time on the ground like the flightless ostrich, emu and kiwi do not have the hind or the first toe at all. Running birds such as courser, plover, and curlew also do not have hind toe. Some birds like duck, quail, partridge, gull etc that have short hind toe do not find it convenient to perch so they spend the night on the ground.