The Audubon Societies 241 



The extremely short period of the Cowbird is perhaps an adaptation to its 

 parasitic habits, for if the young Cowbird hatches ahead of its foster brothers, 

 it has a better chance of getting most of the food and either starving them to 

 death or ousting them from the nest. 



Young birds are assisted in getting out of the shell by what is called the 

 'egg- tooth,' a hard calcareous tubercle which develops on the upper mandible 

 and which is used as the cutting tool in 'pecking' the egg. The bills of all 

 embryo birds are very soft, making such an instrument necessary. This egg- 

 tooth persists for several days after hatching and is quite conspicuous on some 

 birds. Many birds, particularly Grouse and Quail, cut a neat little cap out of 

 the larger end of the egg with this egg-tooth, but others break the shell irregu- 

 larly. Most birds are very careful to remove the empty shells from the nests, 

 either swallowing them or carrying them off to some distance. Birds that 

 have precocial young, however, that do not stay in the nest for any time after 

 hatching, do not bother with the empty shells. 



During the period of incubation the eggs have to be turned once or twice 

 a day so that they will be heated evenly and so that the membranes will not 

 adhere to the shell and prevent the free passage of air to the interior. Some 

 birds turn the eggs with their feet and others with their bills, and usually it is 

 at the time that the female returns from a feeding excursion. The accompany- 

 ing photograph of a Florida Gallinule shows the female bird turning the eggs 

 with her bill. 



(To be continued) 



SUGGESTIONS 



1. What is meant by monogamy among birds and what birds are monogamous? 



2. Have you any definite information upon the mating of the same two birds two years 



in succession? 



3. Have you ever known birds to change mates between broods? 



4. What is meant by polygamy among birds and what birds are polygamous? 



5. Have you ever known House Wrens to be polygamous? Any other species not regu- 



larly so? 

 G. What is meant by communism among birds and what birds are commimistic? 



7. Which sex selects the nesting-site and which sex builds the nest? 



8. What is the duty of the male before and during nest-building? 



9. Have you ever observed a male bird assisting in the building of the nest? To what 



extent? 



10. What determines the nesting material used by birds? The general type of nest? 



11. Have you known of any birds using unusual nesting materials or nesting in unusual 



places? 



12. What determines the selection of the nesting-site? 



13. What is the probable origin of the nest-building habit in birds? 



14. Outline the probable evolution of the more elaborate nests that we know today and 



illustrate the steps by examples of present day nests that represent the different 

 stages. 



15. Why do some birds that lay their eggs in holes build nests and others not? 



