With over 5,000 identified species, it has roughly twice as many species as the largest of the mammal orders. It contains over 110 families, the second most of any order of vertebrates.
Most Perching Birds are smaller. The heaviest and altogether largest are the Thick-billed Raven weighing 1.5 kg and measures 70 cm. The Superb Lyrebird and some Birds-of-Paradise, measures around 110 cm due to very long tails. The smallest Perching Bird is the Short-tailed Pygmy-Tyrant, measuring 6.5 cm and weighing 4.2 grams.
The foot of a passerine has three toes directed forward and one toe directed backwards. This arrangement enables the birds to perch upon vertical surfaces, such as trees and cliffs. The toes have no webbing or joining, but in some cotingas the second and third toes are united at their basal third. The hind toe joins the leg at the same level as the front toes. In other orders of birds the toe arrangement is different. The leg muscle of Perching Birds contains a special adaption for perching. It will automatically tighten and become stiff, if the bird starts to lose hold of the branch on which it is perching. This enables them to sleep while perching without falling off. This is especially useful for birds that develop nocturnal lifestyles.
Most passerine birds develop twelve tail feathers. Certain species have stiff tail feathers, which help the birds balance themselves when perching upon vertical surfaces.
The chicks of passerines are altricial; blind, featherless, and helpless when hatched from their eggs. This requires that the chicks receive a lot of parental care. Most perching birds lay coloured eggs.