MIGRA TION 
113 
leave the work of caring for their young to the foster 
parents. 
The number of eggs that birds lay in their nests varies from 
one to as many as thirty or forty. The time required to 
hatch the eggs varies from ten days to six weeks. Birds 
whose eggs hatch in ten days or two weeks are called altricial 
(al-tri'shal: Latin, altrix, nurse), for such young are hatched 
helpless, blind, 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 substantial 
nest and few of them 
build one. The robin is 
altricial, and the domes¬ 
tic fowl prsecocial (Fig¬ 
ures 102 and 103). 
93. Migration. — Be¬ 
cause 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 
the Northern United States and passes the winder in South 
America, migrating a distance of over five thousand miles. 
In the case of the robin the migration is limited to a short 
flight to the South to some protected swamp provided with 
Figure 101 . — The Robin. 
Sometimes a permanent resident in 
the North. 
