THE BIRDS 



189 



birds sit upon their eggs or incubate them, their develop- 

 ment being dependent upon the warmth afforded by the 

 bird's body. Birds as a rule devote an unusual amount 

 of care to the rearing of offspring and they afford many 

 striking and attractive exhibitions of fidelity and devotion 

 in their family life. The behavior of birds varies greatly, 



FIG. 147. A rookery of nesting birds on the Farallone Islands. (From a 

 group in the museum of the California Academy of Sciences.) 



however, in this regard. Among the lower, or more primi- 

 tive, birds the eggs are laid either in simple, crude nests, 

 or upon bare rocks or soil (see Fig. 147). The labor of 

 incubation in these cases falls entirely upon the female as 

 the associations of the sexes are very temporary and thus 

 stand in marked contrast to the matings which occur in 

 the higher forms. The young of the primitive birds when 



