1588 THE STORY OF THE UNIVERSE 



cording to the available material. And again, since 

 nest-building is obviously an acquired habit, which 

 gradually rewarded the species in the greater success 

 of both parent and offspring during breeding time, 

 it is natural to find it dispensed with in many cases 

 where the nature of the situation rendered no actual 

 nest necessary, or where the birds for some other rea- 

 son have never learned the habit. Some sea-birds, 

 like the auk, simply lay on the rocky ledges of their 

 haunts; some ground-birds simply deposit their eggs 

 on the bare soil. 



Burrowed holes are made by sand-martins, bee- 

 eaters, penguins, kingfishers, and many others. The 

 prairie-owl, living in the burrows of the prairie- 

 dog and of the armadillo, is a well-known example 

 of peculiar habit, and in the first case of curious part- 

 nership. Ground nests, generally of the simplest 

 character, with rough and scanty accumulation of 

 nest material, are made by swans, ducks, geese, fowls, 

 gulls, waterhens, corncrakes, etc. Mud nests, con- 

 structed from damp earth, are well illustrated by the 

 house-swallows, blue-creeper, flamingo, etc. The 

 common singing thrush is well known to make a firm 

 nest of clay and cow-dung mixed with moss. Car- 

 penter-nests, formed with more or less preparation in 

 the holes of trees, are used by woodpeckers and a few 

 other arboreal birds. Platform nests, simply con- 

 sisting of flat seats, are formed by the ring and turtle 

 doves, by eagles, storks, and cranes. In some parts 

 of the Continent the flat nests formed by the storks 

 on the tops of buildings are familiar enough objects. 

 Basket nests are such loosely interwoven construe- 



