THE CLASS BIEDS. 



339 



the time of arrival and departure. In each species, however, 

 the period is fixed, and may he calculated on. Occasionally, 

 age modifies the time of flight or departure, the old leaving 

 before the young. 



Fig. 329. -The Lark. 



444. It is also a remarkable circumstance in the history 

 of birds, that they can leave their nests for so great distances, 



Fig. 331. Colibri, or Humming Bird. 



and yet return without the smallest difficulty or chance of 



mistake. Swallows, for example, year after year (for eighteen 



years, as proved by Spallanzani) return, young and old, suc- 



z 2 



