In this article we will discuss about:- 1. General Characters of Birds 2. Classification of Birds 3. Origin and Diversification.
General Characters of Birds:
1. Birds have spindle-shaped body is highly aero dynamically suitable and covered by feathers. Birds are homoieothermal animals.
2. Small head is placed on a fairly long movable neck.
3. Mouth is provided with a specialised exoskeletal derivative called beak. Teeth are absent in Birds.
ADVERTISEMENTS:
4. Fore limbs are modified as wings, which is powered by strong flight muscles.
5. Hind limbs of birds possess four clawed digits.
6. Eyes of birds possess pecten.
7. Bones become pneumatic to reduce body weight.
ADVERTISEMENTS:
8. Alimentary canal contains gizzard for crushing the food due to absence of teeth.
9. Specialised respiratory system performs double respiration. Air sacs are present in association with lungs.
10. Syrinx is the sound producing organ.
11. Heart of birds is four-chambered. Only right aortic arch is present.
ADVERTISEMENTS:
12. Kidney is metanephric type. Urine is semisolid. Urinary bladder is absent.
13. Only left ovary is present, right ovary absent in birds.
14. These are oviparous animals having telolecithal eggs. Cleavage is meroblastic.
15. During embryonic development four types of extra-embryonic membranes appear. These are chorion, amnion, allantois and yolk sac.
16. Cloaca is divided into three chambers — coprodeum, urodeum and proctodeum.
17. Highly developed nervous system includes brain and sense organs.
Classification of Birds:
As in other chordates, Class Aves is also classified in various ways by various authors. The scheme of classification adopted here is based on Young (1981) edn.
Classification should follow this:
Subclass — Neornithes (Gk : Neos = modern):
ADVERTISEMENTS:
General Characters:
1. Generally beaks of birds are toothless.
2. The short tail has a terminal pygostyle, on which feathers are semi-circularly arranged.
3. Well-developed sternum bears a distinct keel or carina.
4. The metacarpals and carpals unite to form carpometacarpals.
5. Generally 13 or less caudal vertebrae are present.
6. Digits are usually clawed.
This subclass includes two superorders, of which superorder Neognathae is extant.
Superorder — Neognathae:
General Characters:
1. The slender vomer separates the palatine imperfectly.
2. The skull is neognathous type. In this type the palatines are protruded posteriorly and come in contact with the base of the cranium. The palatines remain movably articulated with small pterygoids.
3. Well-developed sternum with keel is present.
4. Wings well-developed. Most of the members possess power of flight.
5. Rectrices are arranged in semicircular manner and helps in flight.
This superorder contains 28 orders of which only one is extinct.
Order — Tinamiformes:
General Characters:
1. Their wings are smaller in comparison to body size. Therefore, they are not good flier.
2. Tail is either absent or very small.
3. Legs are tall and powerful with strong musculature.
4. Pygostyle is degenerated.
Example:
Tinamus (Running birds of South America).
Order — Rheiformes:
General characters:
1. Large flightless birds.
2. Very few feathers are present on head and neck regions.
3. Each leg possesses three digits with claw.
Example:
Rhea.
Order — Struthioniformes:
General characters:
1. Largest living birds in the world.
2. Wings are very small and incapable for flight.
3. Very powerful legs that are armoured, with strong third and fourth digits.
4. Head comparatively small, long neck is devoid of feathers.
5. Keel is present in sternum.
Example:
Struthio camelo (Ostrich, found in Africa and Arabian countries).
Order — Casuariformes:
General characters:
1. Ill-developed wings.
2. Legs are tall and well-developed.
3. Head and neck partially covered by feathers.
4. Legs are provided with three clawed digits.
Example:
Dromicieus (Emu), Casuarius (Cassowary) — both are Australian species.
Order — Dinornithiformes:
General characters:
1. These are insectivorous flightless birds found in New Zealand.
2. Long and slender beaks are present.
3. Eyes are very small in comparison to body size.
4. Nearly degenerated wings are present.
5. Feathers are barbless.
6. Sternum without keel.
Example:
Apteryx (Kiwi).
Order — Podicipediformes:
General characters:
1. Aquatic birds, almost unable to walk on land and are flightless.
2. Rectrices are small, so tail ill-developed.
3. Webbed feet are posteriorly situated.
4. They nest on lakes, laying small number of white eggs in a floating nest.
Example:
Podiceps (Grebes).
Order — Sphenisciformes:
General characters:
1. They lost the power of flight and became specialised for aquatic life.
2. They swim by means of forelimbs, modified into flippers. The feet are webbed.
3. A thick fat layer is present under the skin.
4. Accessory air sacs in the lungs are absent.
5. Bones of the endoskeleton are solid.
Example:
Spheniscus (Penguins — these are mainly confined to the southern hemispheres).
Order — Procellariformes:
General characters:
1. These birds are highly modified for oceanic pelagic life; some of them are very large.
2. Their long narrow wings are specialised for soaring flight.
3. Hook-like beaks are characteristic feature.
4. Tail is small in size.
5. They lay one white egg, often in burrows.
Example:
Diomedea (Albatrosses), Puffinus (Shearwaters), Fulmarus (Petrels).
Order — Pelicaniformes:
General characters:
1. These are aquatic birds, much modified for diving and fishing. They make spectacular dives when fishing.
2. Wings are very long.
3. Webbed feet, each consists of four digits.
4. Beaks are very long.
5. They nest in colonies on rocks or trees.
6. The eggs are usually unspotted and covered with a rough chalky substance.
Examples:
Phalacrocorax (Cormorants), Pelecanus (Pelicans), Sula (Gannets).
Order — Ciconiiformes:
General characters:
1. These are large, long-legged birds, living mostly in marshes.
2. Beak and neck are long in size.
3. All are strong flyers and some of them perform extensive migrations.
4. Nests are usually in colonies and may be used year after year.
5. Eggs are few and unspotted.
Example:
Ciconia (Storks), Ardea (Herons), Phoenicopterus (Flamingoes).
Order — Anseriformes:
General characters:
1. This group of birds are specialised for aquatic life.
2. The characteristic flattened bill is externally covered by hard epidermis.
3. Legs are webbed.
4. Numerous eggs layed that are usually white or pale in colour and the nest is usually built on the ground.
Example:
Anas (Ducks), Cygnus (Swans).
Order — Falconiformes:
General characters:
1. The birds are having sharp, strong and curved bills.
2. Powerful feet are provided with curved clawed digits.
3. These are birds of prey that hunt by day.
4. The retina of the eye contains mainly cones.
5. The eggs, few in number, are usually spotted.
6. Nests are generally made on cliffs, tree tops or other inaccessible places.
Example:
Falco (Kestrel), Aquila (Eagle), Buteo (Buzzard), Naphron (Vulture).
Order — Galliformes:
General characters:
1. Head is markedly smaller in relation to the body size.
2. Powerful legs possess clawed digits.
3. These are mainly terrestrial, grain-eating birds, capable only of short, rapid flights.
4. The palate differs from both that of ratites and of most modern birds— suggesting an early divergence from the stalk.
5. There is marked difference in plumage and sometimes in size between the sexes, i.e., sexual dimorphism present.
6. The eggs are numerous, plain or spotted. Young develop very quickly after birth.
7. Nest usually made on ground.
Example:
Gallus (The jungle fowl of India), Phasianus (Pheasants), Perdix (Partridge), Lagopus (Grouse), Meleagris (Turkey), Numida (Guinea-fowl), Pavo (Peacock).
Order — Gruiformes:
General characters:
1. Terrestrial birds are compressed laterally.
2. Omnivorous birds are living in marshy country-side.
3. These are long-legged birds, can swim, run and dive easily, but are poor flyers.
4. They build simple nests and lay numerous, often dark-spotted eggs.
Example:
Grus (Cranes), Crex (Corncrake), Gallinula (Moorhens), Fulica (Coots), Otis (Bustard).
Order — Charadriiformes:
General characters:
1. They have long legs with three clawed digits in each leg.
2. They possess long bills and feed chiefly on small invertebrates.
3. The birds live mainly on the ground, open watery places or marshes.
4. They are usually gregarious out of breeding reason and are often very numerous on the sea-shore.
Example:
Capella (Snipe), Calidris (Sandpipers), Vanellus (Lapurings), Larus (Gulls), Jacana, Plautus (Auks), etc.
Order — Gaviiformes:
General characters:
1. These are birds of open waters, feeding mainly on fishes.
2. Their legs are webbed.
3. Wings are very weak and they cannot fly.
4. These are known as divers or loons.
Example:
Gavia.
Order — Columbiformes:
General characters:
1. These are tree living, grain or fruit eating birds.
2. Well-developed wings make them good flyers.
3. Head, neck and legs are smaller in comparison to the body size.
4. There is little sexual dimorphism.
5. Nest is usually simple and the eggs normally one or two and white.
6. Young are born under-developed and are nourished by the milk secreted by the crop.
Example:
Columba (Pigeon) (Fig. 1.104), Streptapelia (Dove), Raphus = Didus (The Dodo was a large-sized pigeon of the island Mauritius but was exterminated by man in seventeenth century).
Order — Cuculiformes:
General characters:
1. These are good flyers and possess broad wings.
2. Legs possess four clawed digits — two anteriorly and others backwardly placed.
3. The females lay mostly in the nests of a single foster species.
4. The eggs are strongly mimetic with those of the host.
Example:
Cuculus (Common cuckoo).
Order — Psittaciformes:
General characters:
1. Beaks are hard, upper beak is movably articulated with the skull.
2. The first and fourth digits of the leg are backwardly articulated.
3. They are predominantly vegetarian and make use of the beak for breaking open shells.
4. The eggs are laid in holes and are white and round.
5. The period of parental care after hatching is unusually long (2-3 months).
Example:
Psittacus and Psittacula (Parrot), Probosciger (Cockatoo), Trichoglossus (Loriket), Ara (Mackaw).
Order — Strigiformes:
General characters:
1. These are specialised for hunting at night.
2. The eyes contain mostly rods. The eyes are very large and are directed forwards, and they cannot be moved in the orbit.
3. They detect their prey mainly by sound, and show various specializations in the laterally placed ears.
4. The feathers are so arranged as to make very little noise in flight.
5. Food is swallowed whole.
6. Beaks are well-developed and curved; claws are very sharp and strong.
7. Eggs are white and laid in holes.
Example:
Athene (Little owl), Tyto (Barn owl), Strix (Tawny owl), Nyctea (white owl), Bubo (Eagle owl), Glaucidium (Dwarf owl).
Order — Caprimulgiformes:
General characters:
1. Nocturnal birds feeding on insects taken on wings.
2. Moustache like feathers are present on both side of the mouth.
3. Legs are week and small.
4. Two mottled eggs are laid on the bare ground.
Example:
Caprimulgus (Nightjars), Podargus (Frogmouth).
Order — Apodiformes:
General characters:
1. Wings are very long, composed of a short humerus and long distal segments.
2. Swifts are insectivorous and have very large mouth, adapted for feeding on wings.
3. Tongue of the humming birds is tubular and long.
4. Nests are made in holes or by attaching bracket, sleeve or pocket like structures on trees, cliffs or buildings.
5. The eggs are white, and the youngs are helpless at birth.
Example:
Apus (Swift), Trochilus (Humming bird).
Order — Coliiformes:
General characters:
1. Legs are very small and possess sharp, curved claws in the digits.
2. Tail is extremely long.
Example:
Colius (Mouse-bird).
Order — Trogoniformes:
General characters:
1. Distal end of the beak is hook-like curved.
2. The first and second digits of the leg is backwardly directed while third and fourth digits are anteriorly directed.
Example:
Trogon.
Order — Coraciiformes:
General characters:
1. Beaks are long and well-developed.
2. Three anterior toes are united (syndactyly).
3. These are mainly tropical and often brightly coloured.
4. The nests are usually made in holes and the eggs are white.
Example:
Merops (Bee-eater), Alcedo (Kingfisher).
Order — Piciformes:
General characters:
1. These are highly specialised in climbing. They are insectivorous and wood-boring birds.
2. Bill is very hard and powerful.
3. Tongue is long and protrusible and used for removing insects from beneath the bark.
4. The tail feathers are used to support the bird as it climbs the tree trunk.
5. Second and third digits of the leg is anteriorly placed and fourth and first digits are backwardly directed.
6. Nest is made in a hole in a tree and the eggs are white.
Example:
Picus (Wood pecker), Notharcus (Puff birds).
Order — Passeriformes:
General characters:
1. They are generally four-toed birds. First digit is posteriorly placed and others are anteriorly.
2. The digits are typically arranged to allow the gripping of the perch.
3. They have a complicated nesting behaviour.
4. Eggs are brightly coloured and elaborately marked.
Example:
Corvus (Crow), Sturnus (Starling), Fringilla (Finch), Passer (House- sparrow), Lanius (Shrike), Sylvia (Warbler), Troglodyes (Wren), Hirundo (Swallow).
Origin and Diversification of Birds:
Many characteristics of birds show close resemblance to those of reptiles and, in particular, to the archosaurian diapsids. In the early Triassic period the small pseudosuchians showed the essential characteristics of the bird group, especially those associated with a bipedal habit. From some such form the birds have almost certainly been derived, by a series of changes parallel in many cases to those found in other descendants of the pseudosuchians, such as the crocodiles, dinosaurs and pterosaurs.
The bird plan of structure, originating in the Jurassic period, has become modified to produce the great variety of modern birds. The direction of change can only be described by studying the variety of existing birds today, because there is a dirth of fossil remains for intermediate birds. Modern birds are more thoroughly known than any other groups of animals.
In the reports of those who have studied the variations of birds, there are two distinct opposite tendencies. Many have observed that adaptive radiation has occurred. Birds are found occupying a variety of habitats, with modifications appropriate to each way of life.
Other workers recorded minor differences between races occurring in different areas. The existence of such ‘subspecies’ with a geographical limitation is a stickling characteristic, especially conspicuous in widely distributed species.