Passeriformes

Order of perching or passerine birds consisting of 5,000 to 6,000 species, in 56 families – over half the number of known birds. They are characterized especially by the four-toed foot – three toes directed forwards and one, the hallux, backwards. The hallux is not reversible. This arrangement is well adapted for gripping small twigs or vegetation, and the muscles and tendons act in such a way that as a bird settles onto a perch, the toes tighten against it. This is the only order in which singing is much developed.