298 BIOLOGY FOR BEGINNERS 



selected and woven that the nest often withstands the storms of 

 several seasons, and is repaired and used again, frequently by the 

 same pair that built it. 



Built-up Nests. Robins make a clumsy nest of clay, lined with 

 grass and feathers, placed on the big branches where cats easily 

 reach them. Swallows are much better masons and build clay 

 nests on barns and cliffs, which are very strong and inaccessible. 

 They roll the clay into pellets with the beak and build the walls 

 a little at a time, leaving one layer to dry before adding more, 

 lest it all collapse. The chimney swift (which is not a " swallow " 

 at all) builds a nest of sticks held together by a sticky saliva which 

 hardens into a strong glue. It is used in China to make a sort of 

 edible gelatine; it is from this fact that come the stories of the 

 " edible birds' nests " of that far-off land. These are merely some 

 of the various types of nests. Each species of bird builds its <>\\ n 

 peculiar structure, always in the same way, of similar materials, 

 and in the same kind of location. Yet there seems to be no way 

 in which one generation is taught to build like its ancestors, and 

 when we say it is due to instinct, we have not explained how they 

 learn to construct such perfectly adapted homes. 



Both the nest building and the incubation (sitting) are usually 

 done by the female, though in some species the male helps in both 

 processes. On the other hand the cuckoo avoids either task by 

 laying her eggs in other birds' nests, where the young cuckoos 

 sometimes crowd out their foster brothers. 



Eggs. Reproduction in birds is by means of eggs as has been 

 the usual method in all animals previously studied, but the size, 

 structure, and care of birds' eggs place them on a higher plane of 

 development. The development of birds' eggs requires constant 

 warmth. This necessitates the building of a nest and the constant 

 care of the parent, neither of which is usually required in lower 

 animals. 



Structure. The egg consists of the actual growing point or germ 

 spot at the upper side of the yolk, the yolk surrounded by the 

 " white," this by a double membrane, and this in turn by the shell. 

 The germ cell is fertilized and from it the chick develops, The 



