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 ender 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 odlogists 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-continue:l 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, 
