152 TEXT-BOOK OF ZOOLOGY 



ingly executes a turn to the right. (How is a turn to the left accom- 

 plished ?) 



12. Organs of Sense. — The speed with which birds move points to 

 the possession of sharp-sighted eyes, equally adapted both for far and 

 near vision. We need in this respect only consider the swallows as they 

 rush along with the speed of an arrow, or song-birds searching for 

 insects in the intricate meshwork of bushes and branches. The dispro- 

 portionately large size of the eyes also indicates a sight of more than 

 ordinary acuteness. In its essential features the eye is constructed on the 

 same plan as in mammals (see Part I., p. 12), but besides the upper and 

 lower eyelids it possesses a third eyelid, or nictitating membrane, which 

 extends from the inner angle of the eye over its anterior surface like a 

 transparent veil, and acts as a screen against light of too great intensity. 

 From the fact that many birds are unsurpassed masters of song, it would 

 seem that the organ of hearing is also well developed. There is, how- 

 ever, no external ear, or pinna (excepting in owls, which see). In this 

 connection, too, we must not omit to mention the call and alarm notes 

 used by many birds. The senses of smell and taste are extremely blunt. 

 The bill and tongue function as tactile organs. (Compare the duck and 

 snipe.) 



13. Reproduction and Care of the Brood. — All birds propagate 

 themselves by means of eggs, which, for the sake of protection against 

 injury and excessive evaporation, are surrounded by a calcareous shell. 

 Compare, on the other hand, the eggs of aquatic animals. Most birds 

 build a nest of more or less artistic workmanship, in which the eggs are 

 hatched by the heat of the body of the female or of both parents. 

 (What birds lay their eggs on the bare earth ? Which lays its eggs in 

 the nests of other birds ?) 



Among the song-birds, the male tries to captivate the female by its 

 song : it alone, accordingly, is capable of song. (Compare the lark and 

 the nightingale.) The nests are constructed in many different ways : 

 plaited together (swimming, wading and gallinaceous birds), woven 

 together (golden oriole), felted (chaffinch), chiselled out or built (wood- 

 peckers), walled-up or cemented together (house-martin), dug out or 

 mined underground (sand-martin). (Give other examples.) The young 

 are either helpless and dependent on parental nurture for a period after 

 hatching (aves altrices ; see pigeon) or independent from the moment of 

 liberation from the egg — " precocious " (aves prcecoces ; see domestic fowl). 



14. Change of Habitat. — Dearth of food and cold impel most of our 

 birds, at the commencement of autumn, to migrate to countries further 

 South (migratory birds, or migrants), their routes being determined by 

 the courses of rivers, mountain passes, and ocean straits. They return 



