MIGRATION 



143 



and with little down. Eggs that hatch in from three to 

 six weeks develop well-formed young, able to run around 

 within ten to twelve hours after hatching. These are 

 known as prcecocial (pre-ko'shal : Latin, prae, before; 

 coquere, ripen). Such birds have little need for a sub- 

 stantial nest and few of them build one. The robin is 



Figure 147. — Mourning Dove. 



altricial, and the domestic fowl prsecocial (Figures 1 |.~> 

 and 146). 



137. Migration. — Because they are provided with wings 

 and the power to fly long distances, birds are able to move 

 from one region to another for the purpose of finding food 

 and rearing young. The precise cause of migration is 

 still unknown. Birds in general migrate to a warmer 

 climate in the fall of the year and return to the cooler 

 region in the springtime. In some cases birds cross the 

 equator in migrating. For example, the bobolink nests in 



