The order Passeriformes, commonly known as passerines or songbirds, is by far the largest, with over 6,500 species. It exceeds in number all other bird orders combined. Within the order, the classification is under constant revision. The ecological, morphological and behavioural diversity of this order is extremely high due to the number of species. They are present in all environments and on all continents. Their feet are anisodactyl, with three toes facing forwards and one opposite toe facing backwards.