This order contains an enormous number of birds. Roughly 60% of all birds, or more than 5,000 species, and over 40% or all families of birds are passerines. The taxonomy of this order is still being debated. Contradictions and differences are evident when several sources are consulted.
Birds in this order evolved from ground dwelling forms to tree dwelling birds. All have the typical perching foot, four toes suitable for grasping. The toes are usually free to the base and are joined at the same level to the tarsus. One toe, which cannot be rotated forward, is directed to the rear. The claw of the rear toe is, with few exceptions, larger than that of the middle anterior toe. The grasp of the toes remains firm even when the bird is sleeping. The reason for this is that the flexor tendon and its sheath rest inside one another. Each must be freed before the toes can be extended, allowing the bird to move. These birds use their feet for walking, running or hopping on flat ground, and some are adept at clinging to bark.
Some other common features are short necks and a voice box or syrinx. The syrinx enables the passerines to be the best vocalists and mimics in the animal kingdom. Their calls are more numerous, more varied and more intricate than any other order of birds.
Within the order, there are considerate differences and variations among members. Size may range from 3 ½ inches to 40 inches including tail feathers. Other characteristics offering diversity include the color of plumage, the shape of the bills, habitat and nesting practices. Passerines are land birds, however some do get food from water not far from shore. Most Passerines are good fliers and many are migratory. These birds are found worldwide with the exception of the polar regions and some of the remote oceanic islands.