Definition of Birds
The vertebrate animals whose bodies are covered with feathers as exoskeleton structures are known as birds. Birds possess feathers and only birds possess them.
The birds are so varied in number, adaptations, structures, shapes, food habits etc. that their true classification even upto order is a voluminous work. So, in this text, restriction has been made to classify birds upto sub classes. In this text Parker and Haswell’s classification (1962) of class Aves has been followed.
Sub class- I: Archaeornithes
Sub class- II: Neornithes
General characters of Birds
- Bilaterally symmetrical, homeothermic body (warm blooded)
- Body is divided into head, neck, trunk and tail.
- Body is covered by feathers which are exoskeletal structures.
- Fore limbs have been modified into wings.
- Wing has clawless three digits.
- Hind limb has four digits with claws.
- Hind limb is adapted for walking, swimming and for perching.
- Except uropygeal gland near the tail there is no other cutaneous gland.
- Skull monocondylic.
- Except fossil bird, no modern bird has any tooth.
- Vertebrae heterocoelous (vertebrae with centra having saddle-shaped articulations)
- Ribs double headed, and provided with uncinate process.
- Sternum has modified into a boat shaped keel for muscle attachment.
- Clavicle and interclavicle are united to form a U-shaped fercula.
- Carpals and metatarsals join to form caprometa carpals.
- Tarsal and metatarsals join to form tarsometatarsals.
- Syrinx acts as voice apparatus.
- In the alimentary canal a crop is present.
- Neck is highly mobile, some becomes very long.
- One pair of lungs present, lungs remain attached with thoracic ribs.
- Lungs are provided with air sacs.
- Heart completely four chambered. RBC large, nucleated and biconvex.
- Only right aortic arch present.
- Kidney metanephric type.
- Left ovary functional; right ovary vestigial.
- Fertilization internal, they lay yolky eggs, eggs are covered by shells made of calcium carbonate.
- In the embryo amnion, chorion, allantois and yolk sac are present as extra embryonic membranes.
- Large brain with 12 pairs of cranial nerves.
Classification with reasons and examples
Sub Class I. Archeornithes
- Elongated body with all developed wings by which they could fly for a shorter distance.
- Tail long and pointed like a lizard-tail and was provided with feathers.
- Each hand bearing three unfused and clawed fingers.
- Tail with 18-20 free caudal vertebrae.
- Sternum without a keel.
- Each limb had 3 clawed digits.
- Teeth present on both jaws.
- Abdominal ribs present.
- Sternum was without any keel.
- Cerebellum small.
Sub class II: Neornithes
- Modern birds
- Both living and extinct birds are present under the sub class.
- Wings well developed can fly well.
- Tail short and small.
- Tail feathers (rectrices) are arranged as a fan.
- The digits of forelimb generally without any claws.
- Except some fossil birds modern birds have no teech.
- Vertebrae heterocaelous type.
- Abdominal ribs absent.
- Teeth absent except in some fossil birds.
- Few caudal vertebrae free. Rest fused into a pygostyle.
- Sternum with well developed keel.
- Thoracic ribs with uncinate process.