THE STRUCTURE AND ACTIVITIES OF BIRDS 357 



parents (Fig. 228). Often the young cowbirds are stronger and 

 thus starve out the rightful owners of the nest. 



Many birds nest in marshy situations and are consequently 

 very seldom seen by most people. They are the marsh wrens, 

 gallinules, rails, grebes, terns, loons, bitterns, and many others. 



Fig. 236 B. 



-Two young red-tailed hawks 27 days old and almost 

 ready to fly. (Photo, by Hegner.) 



Their nests are built of dry rushes and other plants and sus- 

 pended among the rushes above the water, or are made on the 

 soggy ground of decaying vegetation (Figs. 220, 230, 231). 



Precocial and Altricial Birds. — It is necessary to distinguish 

 between precocial and altricial birds in discussing the eggs and 



