Being primarily designed for flying, birds have had access to the treetops in a way that few other animals had, and finding that they were safer there than on the ground they soon became perfectly adapted to this new habitat. As a result many woodland birds of the Posthomic have developed feet with curved opposable toes that are ideal for gripping branches. In one species, the tree goose, these toes have been reduced to two. They are permanently curved and enable the bird to hang upside down without effort. Because of the bird's size and weight, this attitude is much easier to maintain over long periods than an upright stance, and it has taken to spending long periods roosting in this position.