A passerine is a bird of the order Passeriformes, which includes more than half of all bird species. They are perching birds. Most of them are small in size, and most can sing very well. Their main suborder is the Passeri, the songbirds.

Adaptations 
The passerines have a number of adaptations which, taken together, have made them highly successful, with many species around the world.

Perching. All passerines have four toes on each foot. Three of the toes point forwards. The other toe points backwards. This means that passerines can easily hold onto trees or rocks. Because of this, they are also known as perching birds.  A tendon in the rear of the leg makes the foot curl and grip when the bird lands on a branch. This is more than just a convenience for the birds: it is a tendon locking mechanism so they can sleep without falling off.
All passerine chicks are born in an undeveloped state (altricial): they need parental help. Their eyes are closed, and they are bald, with little or no down. They cannot leave the nest, and must be fed by the parents. The advantage of this is that their brains grow larger during the fledgling stage. In birds which leave the nest early, their brains depend mainly on material in the egg. Passerines have significantly large brains compared to body size, and many passerines are comparatively intelligent.
The use of song for communication. This is the most obvious adaptation.
Almost all passerines have 12 tail feathers. This is a basal trait, a feature inherited from their past. It leads biologists to think this large group originally had one common ancestor.

Evolution 
Evidence on their evolution comes from comparative anatomy, the fossil record and molecular biology. 

It is thought that the first passerines evolved in the southern continent of Gondwana in the Palaeogene. This might have taken place around the Upper Palaeocene some 60–55 mya. The latest opinion on passerine evolution is that that passerines originated on the Australian landmass ∼47 mya.

The early fossil record is poor because the first Passeriformes were on the small side, and their delicate bones did not preserve well. Two specimens from Queensland, Australia are fossil bone fragments clearly recognizable as passeriform. They are two species of about 10 and 20 cm in overall length. This proves that some 55 mya, barely into the early Eocene, early perching birds were recognizably distinct. It also suggests that the origin of this great order of birds was in the southern continent.

Taxonomy 

There are three sub-orders: Acanthisitti, Tyranni and Passeri. There are only two bird species in the sub-order Acanthisitti. They are known as New Zealand wrens. The sub-order Tyranni are also known as the suboscines. Many of these birds are found in tropical areas. The sub-order Passeri has the most bird species. They are also known as the oscines.

Suborder Acanthisitti
 Acanthisittidae: New Zealand "wrens"

Suborder Tyranni
Suboscines
 Infraorder Eurylaimides broadbills and allies
 Infraorder Tyrannides - New World suboscines

Suborder Passeri
Songbirds or oscines
 Basal Passeri – the oldest 'true' song birds. They live in Australia.
 Menuridae: lyrebirds
 Atrichornithidae:  scrub-birds
 Superfamily Meliphagoidea – mainly insectivores and nectarivores, distribution centered on Australo-Melanesian region extending into surroundings, notably the Pacific.
 Superfamily Corvida – a highly diverse group of global distribution, but most plentiful in the Australasian region and surroundings. The oldest truly globally successful group of passerines, they include among them what may well be the most intelligent and the most spectacular of the order.
 Melanocharitidae:  berrypeckers and longbills.
 Callaeidae: New Zealand wattlebirds
 Stitchbirds
 Cnemophilidae: satinbirds
 Neosittidae: sittellas
 Vireonidae: vireos
 Campephagidae: cuckoo-shrikes and trillers
 Pachycephalidae: whistlers and allies
 Oriolidae: orioles and figbirds
 Paramythiidae: tit berrypecker and crested berrypecker.
 Artamidae: woodswallows, butcherbirds, currawongs and Australian Magpie
 Malaconotidae: puffback shrikes, bush shrikes, tchagras and boubous
 Platysteiridae: wattle-eyes
 Aegithinidae: ioras
 Pityriaseidae: Bornean bristlehead.
 Prionopidae: helmetshrikes and woodshrikes
 Vangidae: vangas
 Dicruridae: drongos
 Monarchidae: monarch flycatchers
 Rhipiduridae: fantails
 Paradisaeidae: birds of paradise
 Corcoracidae: white-winged chough and apostlebird
 Laniidae: shrikes
 Corvidae: crows, ravens and jays
 Corvoidea (?)
 Passeri
 Ptilonorhynchoidea: bowerbirds and Australian treecreepers
 Logrunners and pseudo-babblers
 Petroicidae: Australian robins
 Rockfowl, rock-jumpers and rail-babblers
 Regulidae: kinglets
 Hyliotas
 Irenidae: fairy-bluebirds
 Chloropseidae: leafbirds

Infraorder Passerida
 Superfamily Sylvioidea – mostly insectivores, distribution centered on the Indo-Pacific region. Few occur in the Australian region and fewer still in the Americas. Usually sleek and drab birds, few have pronounced sexual dimorphism.
 Alaudidae: larks
 Hirundinidae:  swallows and martins
 Phylloscopidae: leaf-warblers and allies.
 Aegithalidae: long-tailed tits or bushtits
 Cettiidae: ground-warblers and allies
 Megaluridae: grass-warblers and allies
 "Bernieridae": Malagasy warblers. A new family.
 Acrocephalidae: marsh- and tree-warblers
 Pycnonotidae: bulbuls
 Cisticolidae: cisticolas and allies
 Sylviidae: "true warblers" and parrotbills.
 Zosteropidae: white-eyes
 Timaliidae: (Old World) babblers.
 Sylvioidea (?)
Superfamily Muscicapoidea – mostly insectivores, near-global distribution centered on Old World tropics. One family endemic to Americas. Nearly absent (except introductions) from the Australian region. Usually rather stocky for their size, most are quite dark and dull though Sturnidae are commonly iridescent and/or colorful. Sexual dimorphism often absent, sometimes pronounced.
Cinclidae: dippers
Muscicapidae: Old World flycatchers and chats.
Turdidae: thrushes and allies.
Buphagidae: oxpeckers. Formerly usually included in Sturnidae.
Sturnidae: starlings and possibly Philippine creepers.
Mimidae: mockingbirds and thrashers

 Superfamily Certhioidea - wrens and allies 
 Sittidae:  nuthatches
 Tichodromadidae: wallcreepers
 Certhiidae:  treecreepers
 Salpornithidae: spotted creeper
 Troglodytidae: wrens
 Polioptilidae: gnatcatchers
 Superfamily Passeroidea – mostly herbivores including many seed-eaters, near-global distribution centered on Palearctic and Americas. Includes the Nine-primaried oscines (probably a subclade). A very high proportion of colorful and highly sexually dimorphic forms.
 Passeridae: true sparrows
 Prunellidae: accentors
 Motacillidae: wagtails and pipits
 Urocynchramidae Przewalski's finch
 Estrildidae:  estrildid finches (waxbills, munias, etc.)
 Ploceidae: weavers
 Viduidae: indigobirds and whydahs
 Nine-primaried oscines:
 Peucedramidae: olive warbler
 Fringillidae: true finches and Hawaiian honeycreepers
 Icteridae: grackles, New World blackbirds, and New World orioles
 Parulidae: New World warblers
 Thraupidae: tanagers and allies
 Cardinalidae: cardinals
 Emberizidae: buntings and American sparrows
 Coerebidae: bananaquit
 Passerida: (?)
 Panurus: bearded reedling ("bearded "tit")
 Paroidea: titmice and allies
 Paridae: tits, chickadees and titmice
 Remizidae: penduline tits
 Stenostiridae: stenostirids ("flycatcher-tits")
 "Bombycilloidea" waxwings and allies
 Bombycillidae: waxwings
 Dulidae: palmchat
 Ptilogonatidae: silky flycatchers
 Hypocoliidae: Hypocolius
 Mohoidae
 "Dicaeoidea" sunbirds and flowerpeckers
 Nectariniidae: sunbirds
 Dicaeidae: flowerpeckers
 Promeropidae: sugarbirds

References