It is estimated that the total number of bird species that have existed is 154,000, of which 9,600 are alive today. Wetomore (1960) described the 34 orders, out of which 27 orders are of living birds (2 orders of these have recently become extinct) and 7 orders of fossil birds. Class Aves is divided into 2 subclasses. The classification is adopted from Storer and Usinger (1957).

Subclass I. Archaeornithes:

(Gr., archios = ancient + ornithos = bird)

1. Extinct Jurassic birds of Mesozoic age, about 155 million years ago.

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2. Wings with little power of flight.

3. Teeth embedded in sockets were present in both jaws.

4. Tail long with more than 13(18-20) caudal vertebrae bearing rectrices arranged in two lateral rows.

5. Pygostyle was absent.

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6. Vertebrae were amphicoelous. Thoracic ribs without uncinate process. Abdominal ribs present.

7. Sternum without keel.

8. Three carpals and metacarpals were free, and three clawed digits.

Order Archaeopterygiformes:

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Examples: Archaeopteryx lithographica and Archaeornis siemensi found in the Upper Jurassic limestone of Solenhofen in Bavaria. Each was about the size of a crow.

Subclass II. Neornithes:

(Gr., neos = modern + ornithos = birds)

1. Living as well as extinct birds.

2. Wings usually well-developed used for flight with a few exceptions.

3. Tail is short, ends in a pygostyle. Rectrices are arranged in a semicircle around the pygostyle.

4. Teeth are absent except in few fossil birds.

5. Vertebrae are heterocoelous in living forms.

6. Caudal vertebrae are 13 or less. Few caudal vertebrae free and rest fused into a pygostyle.

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7. The sternum is well developed and usually provided with a keel or carina.

8. The metacarpals are fused with the distal carpals to form a carpo-metacarpus.

9. Except in one case, not more than two digits of the hand bear claws. In nearly all cases claws are absent in the hand.

10. Thoracic ribs with uncinate processes. Abdominal ribs are absent.

The subclass is divisible into four super orders:

Superorder I. Odontognathae:

(Gr., odontos = teeth)

1. Extinct upper Cretaceous birds.

2. Teeth present on jaws, advantageous for catching fish.

3. Brain is of avian type rather reptilian (Edinger).

Order 1. Hesperornithiformes:

1. Large marine diving and swimming birds.

2. Sharply pointed teeth present in grooves and not in sockets.

3. Premaxillae without teeth.

4. Vertebrae amphicoelous.

5. Sternum without keel.

6. Shoulder girdle was much reduced.

Examples: Hesperornis, Enaliornis, Baptornis and Neogoeornis.

Order 2. Ichthyornithiformes:

1. Wings well developed.

2. Brain was of avian type.

3. Whether teeth were present is not definite.

4. Neck vertebrae amphicoelous.

5. Sternum with well-developed keel.

Examples: Ichthyornis, Apatornis belonged to upper Cretaceous of North America.

Superorder II. Palaeognathae or Ratitae:

(Gr., palaios = old + gnathos = jaw; L., ratis = raft)

1. Large sized, flightless, walking or running birds.

2. Wings are reduced or absent. Feathers without interlocking mechanism.

3. Tail feathers irregularly arranged or absent. Teeth are absent.

4. Except Tinamus and Kiwi, oil gland is lacking.

5. Skull is palaeognathous (dromaeognathous), i.e., vomer is broad and large and located in between palatines.

6. Quadrate is single headed.

7. Sternum devoid of keel or carina, or flat, raft-like.

8. Uncinate processes in ribs are absent or vestigial.

9. Tail vertebrae are free. Pygostyle small or absent.

10. Coracoid and scapula are small and fused at an obtuse angle.

11. Clavicles are small or absent.

12. Except Rhea and Emu, ilium and ischium are not united posteriorly.

13. Pectoral muscles not well developed.

14. Syrinx is absent.

15. Penis in male is large and erectile and female possesses a clitoris.

16. Youngs are precocious.

17. Ratites are not found in India. They are classified into 7 orders.

Order 1. Struthioformes:

(Gr., struthio = ostrich + form)

1. Large flightless terrestrial birds.

2. Feathers without aftershaft.

3. Head, neck and legs sparsely feathered.

4. Head is comparatively small and the neck is very long and flexible.

5. The beak is short and broad.

6. Only two toes (3rd and 4th) on each foot.

7. Pubic symphysis present.

Examples: Struthio camelus (True ostriches of Africa and Western Asia).

Order 2. Casuariiformes:

(NL. casuarius = genus of cassowary + form)

1. Large flightless terrestrial birds.

2. Head is sparsely feathered, while neck and body densely feathered. Head bears a comb-like structure.

3. Feathers with after shaft nearly equal to shaft.

4. Wings small or rudimentary.

5. Three toes with typical claws on each foot.

Examples: Dromaeus (Emus of New Zealand), Casuarius (Cassowaries of Australia and New Guinea).

Order 3. Aepyornithiformes:

1. Flightless, terrestrial extinct elephant birds.

2. Wings vestigial or tiny.

3. Legs powerful with 4 toes.

4. Eggs were surprisingly large.

Examples: Aepyornis titan, Mulleornis (Giant elephant birds of Africa).

Order 4. Dinornithiformes:

1. Giant flightless, terrestrial, extinct birds (extinct about 700 years ago).

2. Feathers with large aftershaft.

3. Wings almost absent.

4. The beaks were short.

5. Legs massive bearing 4 toes.

Examples: Dinornis maximus (Moas of New Zealand).

Order 5. Apterygiformes:

(Gr., a = not + pteryx = wing + form)

1. Flightless terrestrial birds.

2. Feathers fluppy hair-like without after shafts.

3. Wings are rudimentary or degenerate.

4. The beak is long and slender with nostrils at the tip of the maxilla.

5. The eyes are small.

6. The neck and legs are comparatively short.

7. Eggs are relatively largest of all the living birds.

8. Nocturnal, feeding on worms and insects.

Examples: Apteryx (Kiwis of New Zealand).

Order 6. Rheiformes:

(Gr., Rhea = mother of Zeus + form)

1. Flightless terrestrial and swift-running birds.

2. Head, neck and thighs are feathered.

3. Wings are better developed.

4. Legs bear 3 toes with heavy and typical claws.

5. Ischia form a ventral symphysis.

Examples: Rhea americana (American ostrich found in South America).

Order 7. Tinamiformes:

(NL. tinamus = genus of tinamou)

1. Small, terrestrial, essentially cursorial with poor power of flight.

2. Wings are short and rounded.

3. Sternum is keeled.

4. Vegetarian.

Examples: Tinamus (Tinamou), Rhyncotus.

Superorder III. Impennae:

Order 1. Sphenisciformes:

(Gr., spheniscus = wedge + form)

1. Modern, aquatic, flightless birds.

2. Wings paddle-like or flippers.

3. Feet are webbed.

Examples: Aptenodytes (Penguins).

Superorder IV. Neognathae:

(L., carina = a keel)

1. Modern, usually small, flying birds.

2. Wings well developed.

3. Feathers having interlocking mechanism.

4. Rectrices are present and regularly arranged.

5. Pterylae are regular, while irregular in ratites.

6. Oil gland is present.

7. Skull is neognathus, i.e., vomer short and palatines meet with each other.

8. Quadrate is double-headed.

9. Sternum with a well-developed keel.

10. Ribs with uncinate processes.

11. Pygostyle present.

12. Scapula and coracoid unite at a right angle.

13. Ilium and ischium are posteriorly united.

14. Pectoral muscles well developed.

This superorder includes several orders which are as follows:

Order 1. Gaviiformes:

(L., gavia = sea mew + form)

It includes marine birds.

1. Legs are short and at the posterior end of the body.

2. Tail comprises 18-20 short stiff feathers.

3. Toes are fully webbed.

4. Patella reduced.

5. Powerful in flight.

Examples: Gavia immer (Common loon).

Order 2. Podicipeformes or Colymbiformes:

(Gr., kolymbos = diving bird)

1. Freshwater divers.

2. Feet are lobed.

3. Tail comprises a tuft of down feathers.

4. Patella is large.

5. Tarsus is compressed.

Examples: Podicipes, Podilymbus.

Order 3. Procellariiformes:

(L., procella = a tempest + form)

1. Nostrils are tubular.

2. Horny sheath of bill consisting of several plates.

3. Skull with large nasal glands.

4. Hind toe is vestigial or none.

5. Feathers compact and oily in texture.

6. Wings are long and narrow.

7. Strictly marine, nest usually on islands.

Examples: Diomedea (Albatrosse), Puffinus, Procellaria (Petrels), sheawaters.

Order 4. Pelecaniiformes:

(L., pelicanus = pelican + form)

1. Aquatic fish-eating and colonial nesters.

2. Nostrils are vestigial or absent.

3. Presence of a gular pouch on the throat.

4. Four toed webbed feet.

5. Beaks are long with wide gapes for catching and swallowing the struggling prey.

Examples: It includes pelicans, darters, gannets, and cormorants. Pelecanus (Pelican), Phalacrocorax (Cormorant), Anhinga (Indian darter).

Order 5. Ciconiiformes:

(L., ciconia = a stork + form)

1. Long snake-like neck and long-legged birds.

2. Feathers are decorative.

3. Beaks abruptly decurved at middle in flamingos.

4. Toes are without web.

5. Young born naked.

6. Food comprises fish and other aquatic animals.

Examples: It includes marshy wading birds: Ardea (Great blue heron), Phenicopterus (Flamingo), Bubulcus (Egret), Nycticorax (Night Heron), Platalea (Spoonbill), Ciconia (Stork).

Order 6. Anseriformes:

(L., anser = goose + form)

1. Beak or bill is broad, covered with soft cornified epidermis.

2. Margins of the beak containing many transverse horny ridges (lamellae).

3. Tongue is fleshy.

4. Legs are short and feet are webbed.

5. Tail is usually short, consisting of many feathers.

Examples: Aquatic birds such as Ducks, Geese, Swans. Anas (Wild duck), Nettion crecca (Common teal), Anser indica (Bar-headed goose), Cygnus (Swan).

Order 7. Falconiformes:

(L., faico = falcon + form)

1. Beak is short and curved at the tip.

2. Mandibles are sharp edged.

3. Feet usually with sharp curved claws adapted for grasping and holding the prey.

4. Diumal and strong fliers.

Examples: Vultures, Kites, Hawks and Falcons. Milvus migrans (Pariah kite), Haliaster Indus (Brahminy kite), Astur badius (Sparrow hawk), Pseudogyps (Vulture), Falco pelegrinus (Falcon), Aquila (Eagle).

Order 8. Galliformes:

(L., gallus = a cock + form)

1. Terrestrial game birds with short and powerful flight.

2. Beak is short.

3. Feathers with after shaft.

4. Feet usually massive adapted for scratching and running.

5. Graminivorous.

Examples: Game birds. Gallus (Jungle fowl), Pavo cristatus (Peacock), Coturnix (Quail), Francolinus (Grey partridge), Phasianus (Pheasants).

Order 9. Gruiformes:

(L., grus = crane + form)

1. Small, medium and large-sized birds.

2. Flightless or weak fliers or strong fliers.

3. Feathers with aftershaft.

4. Legs long and beaks heavy.

Examples: Crane-like wading birds. Grus (Crane), Fulica atra (Common coot), Antegone antegone (Sarus crane), Choriotis (bustard), rail.

Order 10. Diatrymiformes:

1. Large flightless extinct birds.

2. Wings atrophied.

3. Massive head, beak and neck.

4. Foot bears 4 toes.

Example: Diatryma (Extinct).

Order 11. Charadriiformes:

(NL., charadrius = genus of plovers + form)

1. Shore dwelling aquatic birds.

2. Fore- and hind limbs are slender and enormously elongated.

3. Toes usually webbed.

4. Beaks mud probing.

5. Feathers are dense and firm.

Examples: Lobivanellus indicus (Red wattled lapwing), Hydrophasianus chirugus (Pheasant tailed jacana), Tringa glariola (Sand piper), Capella (Snipe), Larus (Gull), etc.

Order 12. Columbiformes:

(L., columba = dove + form)

1. Skin thick and soft.

2. Beak usually short and slender.

3. Tarsus usually shorter than toes.

4. Crop large producing “pigeon milk” to feed young ones.

5. Graminivorous and frugivorous.

Examples: Columba livia (Blue rock pigeon), Goura cristata (Crowned pigeon), Streptopelia risoria (Ring turtle dove), Streptopelia chinensis (Spotted dove), Raphus (extinct dodo), etc.

Order 13. Cuculiformes:

(L., cuculus = cuckoo + form)

1. Toes 2 in front and 2 behind, out hind toe reversible.

2. Feet not adapted for grasping.

3. Tail long and beak moderate.

4. Many cuckoos are parasitic, the female laying eggs in nest of other birds.

Examples: Cuculus canorus (Cuckoo), Eudynamis scolopaceous (Koel), Centropus sinensis (Crow pheasant).

Order 14. Psittaciformes:

(L., psitacus = parrot + form)

1. Feathers are green, blue, yellow or red.

2. Beak stout, narrow, sharp-edged and hooked at the tip.

3. Upper jaw is movably articulated with the skull.

4. Feet zygodactylus, i.e., two toes in front and two toes behind.

5. Outer toe is not reversible.

Examples: Psittacula eupatria (Large Indian Parakeet), Psittacula krameri (Green parrot), Melopsittacus (Budgeriger).

Order 15. Strigiformes:

(Gr., strix = owl + form)

1. Head large and rounded.

2. Eyes large and directed forwards. Each in a disc of radial feathers.

3. Retina contains principally the rods.

4. Ear opening large, often with flap-like cover.

5. Beak short, feet with sharp claws adapted for grasping.

6. Nocturnal and predators.

Examples: Bubo bubo (Great homed owl), Ketupa zeylonensis (Brown fish owl), Tylopus.

Order 16. Caprimulgiformes:

(L., caprimulgus = goat sucker + form)

1. Beak is small and delicate.

2. Mouth is wide and margined with long bristle-like sensory feathers.

3. Feathers are soft and lax.

4. Legs and feet small and weak, not adapted for grasping.

5. Nocturnal and insectivorous.

Examples: Caprimulgus (Goat sucker), Chordeiles (Night hawks), Phaeloenoptilus (Whippoor wills).

Order 17. Apodiformes (Micropodiformes):

1. Usually small birds.

2. Legs are very short and feet very small.

3. Wings very long formed of short humerus and long distal segments.

4. Beak is small and weak or slender with long tubular tongue.

Examples: Swifts (Apus) are insectivorous. Humming birds (Trochilus).

Order 18. Coliformes:

1. Small passerine birds.

2. First and fourth toes are reversible.

3. Tail is very long.

4. Insectivorous and frugivorous.

Example: Colius (Mouse birds or Colies) is confined to Africa.

Order 19. Trogoniformes:

1. Beak short and stout with bristles at the base.

2. Feet small and weak.

3. Feathers brilliant in colour, often green.

Example: Trogoti.

Order 20. Coraciformes:

(Gr., korax = crow or raven + form)

1. Beak strong.

2. Third and fourth toes fused at the base.

Examples: Kingfishers and their allies. Halcyon smyrnensis (White-breasted kingfisher), Ceryle rudis (Pied kingfisher), Dichoceros bicornis (Great hornbill), Tochus birostris (Grey hornbill), Upupa epops (Hoopoe).

Order 21. Piciformes:

(L., picus = woodpecker + form)

1. Tail feathers are stiff with pointed tips.

2. Beaks stout and powerful.

3. Protrusible tongue is roughened or with barbs at the tip.

4. Two toes in front and two or one behind, not reversible.

Examples: Woodpeckers and their allies. Dryobates (Yellow front pied woodpecker), Brachypternus bengalensis (Golden beak woodpecker), Dendrocopos, etc.

Order 22. Passeriformes:

(L., passer = sparrow + form)

1. Toes three in front and one behind, adapted for perching.

2. Beaks are adapted for cutting.

Examples:

Passer domesticus (Common + house sparrow), Corvus splendens (Common house crow), Corvus macrorhynchos (Indian jungle crow), Acridotheres tristis (Common myna), Saxicoloides fulicata (Indian robin), Oriolus oriolus (Golden oriole), Dicrurus macrocercus (Black drongo or King crow), Ploceus philippinus (Weaver bird or Baya), Muscicapa (Flycatchers), Bulbul (Molpestes), Swallos (Hirundo), Pica (Magpie), Loxia (Crossbill), Sturnus (Starling), Alauda (Larks), Fringilla (Finches), Anthus (Pipits), Motacilla (Wagtails), Certhia (Tree-creepers), Paras (Tits), Lanius (Shrikes), Sylvia (Warblers).

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