INCUBATION OP BIRDS. 387 



and have least power of resisting the effects of the privation of 

 it. They produce therefore greater heat; the temperature of 

 their bodies rises to 108 or even to 112 ; and the feathers with 

 which they are covered are very useful in preventing the loss of 

 heat from their bodies, when they rise to great heights in the 

 air, or when they are partly or wholly immersed in water. As 

 in the Mammalia, the organ of voice depends for its action on 

 the respiratory apparatus ; but the sound is produced at the 

 bottom, and not at the top, of the trachea. As its peculiar 

 structure has been elsewhere described (ANIM. PHYSIOL., 685), 

 it is not requisite to dwell upon it here. 



350. Birds are oviparous, and do not possess, like the ani- 

 mals of the preceding class, mammary glands to suckle their 

 young. The duration of the incubation (or of the time which 

 the young Bird takes to develop itself in the interior) varies in 

 different species, but is nearly constant in each ; in the Humming- 

 bird, the smallest animal of this class, it is only twelve days ; 

 in Canaries, in a domesticated state, it is from fifteen to eighteen 

 days ; twenty-one days for Fowls ; twenty-five for Ducks ; and 

 from forty to forty-five for Swans. A certain degree of heat is 

 necessary for this operation ; that of the sun is sufficient to 

 hatch the eggs of some Birds inhabiting countries between the 

 tropics ; but generally it is quite otherwise ; and to keep up in 

 the eggs a suitable temperature, the mother covers them with her 

 body, and places them in a bed adapted to shelter them. Some 

 Birds are satisfied with making for this purpose, a rounded 

 cavity in the earth or sand; but most display, in the con- 

 struction of this kind of cradle, an admirable skill and art ; and 

 a point not less remarkable, is the regularity with which suc- 

 cessive generations execute the same labours, and build their 

 nests in exactly the same mode, even when the circumstances in 

 which they have been placed have not permitted them to see any 

 others or to take lessons from their parents. They are guided 

 by an admirable instinct, which leads them to take a number of 

 precautions, of whose utility they have no previous knowledge 

 derived from experience. The sides of the nest are usually 

 formed of small flexible twigs, sometimes cemented with earth 

 which has been tempered with the gummy saliva of the animal ; 



