Indian Birds

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in various parts of the body, whence they fall over and evenly cover the adjoining naked interspaces or apteria. A study of the arrangement of the feather tracts (pterylosis) which varies in the different orders, families and even species is of great importance in determining the natural relationships of different birds.

The feathers covering the body of a bird fall into 3 classes : (1) The ordinary outside feathers known as Contour feathers or pemue, whether covering the body as a whole or specialised as pinions or flight feathers (remiges) or as tail feathers (rectrices) which serve as rudder and brake ; (2) the fluffy Down feathers or plumules hidden by the Contour feathers and comparable to flannel underclothing, whether confined to nestlings or persisting throughout life ; (3) the hair-like Filo-plumes which are hardly seen until the other feathers have been plucked off. They are particularly noticeable, for instance, in a plucked pigeon.

The body temperature of birds is ...

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so they need not come under discussion here. The forelimbs of Birds, which correspond to human arms or to the fore-legs of quadrupeds, have been evolved to serve as perfect organs of propulsion through the air. Many of their larger bones are hollow and often have air sacs running into them, which, as mentioned above, function principally as accessory respiratory organs. This makes for lightness without sacrificing strength, and is a special adaptation to facilitate aerial locomotion. Modifications in the structure of the breast bone, pectoral girdle and other parts of the skeleton, and the enormously developed breast-muscles enable a bird to fly in the air. It has been estimated from ...

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