68 BIRDS' EGGS. 



which we can give no satisfactory reason. Thus the 

 Crested Flycatcher's strange custom of using a cast 

 snake-skin in its nesting materials probably originated 

 with the birds in the tropics, where it is still followed 

 by nearly related species of Crested Flycatchers. With 

 them there may be a reason for this habit, but with our 

 bird, living as it does under entirely different conditions, 

 it is doubtless only an inheritance, surviving even when 

 the necessity for it has ceased to exist. 



Eighth, change of habit. Some birds are influenced 

 by changes in their surroundings, and alter their nesting 

 habits when it proves to their advantage to do so. 

 Chimney Swifts, who have exchanged hollow trees, in 

 which they were exposed to their natural enemies, for 

 the comparative safety of chimneys, are good examples. 

 But a far better one is given by that prodigy in feathers, 

 the House Sparrow. Is there any available site in which 

 this thoroughly up-to-date bird will not place its nest? 

 It has taken possession of even the hollow spaces about 

 certain kinds of electric lamps, and has been observed 

 repairing its nest at night by their light ! 



The Eggs. — Usually, little time is lost between the 

 completion of the nest and the laying of the eggs. The 

 number of eggs composing what oologists term a full 

 set or clutch ranges from one to as many as twenty. At 

 the time of laying, the ovary contains a large number of 

 partly formed eggs, of which, normally, only the required 

 number will become fully developed. But if the nest be 

 robbed, the stolen egg will frequently be replaced. The 

 long-continued laying of our domestic fowls is an instance 

 of this unnatural stimulation of the ovary. Doubtless the 

 most remarkable recorded case of egg-laying by a wild 

 bird is that of a High-hole or Flicker, who, on being regu- 

 larly robbed, laid seventy-one eggs in seventy-three days ! 



The eggshell is composed largely of carbonate of lime, 



