CLASS AVES^(BIRDS). -525 



ing. The eggs' must ^fertilized within the body, because 

 the "white" of the egg and the shell are added by the 

 oviduct after fertilization. Because of their warm-blood- 

 edness the young must have warmth in which to develop. 

 This is usually supplied by the body of the mother in 



FIG. 132. Nestling marsh hawks (Circus cyaneus). Year-Book Department of Agri- 

 culture. 



Questions on the Figure. What are the nesting and breeding 

 habits of the marsh hawk? Are the young precocial or altricial? 



sitting on the eggs. The number of eggs laid varies from 

 one to twenty or more. The time necessary for hatching 

 varies roughly with the size of the eggs, from ten to fifty 

 days. Most birds rear only one brood each year; some 

 rear two, and the English sparrow may produce five or 

 six in a season. 



The nest itself varies from a mere hole in the sand or 



