126 ON THE INSTINCTS OF BIRDS. 
warmth, cleanliness, and stability ; but the undeniable 
fact that birds, when taken very young, even before 
they can see, and brought up in confinement, do 
sometimes construct nests, is alone sufficient to refute 
this opinion..- 
The Sparrow-Hawk and Kestril often make use of 
the deserted habitation of the Magpie as a receptacle 
for their eggs, and the Sparrow frequently takes 
forcible possession of the rustic dwelling of the 
House-Martin for the same purpose. Why, then, 
are they never known to build nests similar to those 
which they thus appropriate to themselves? and why 
does not the Cuckoo, which is always brought up in 
the nest of some other bird, construct one itself» * 
The reason is obvious: the act of nidification is not 
regulated by observation or instruction, but is under 
the immediate direction of instinct. 
Guided by that mysterious power, individuals of 
the same species, under like circumstances, always 
adhere to the same style of architecture. Thus some 
of the smaller birds, which produce a large number 
of eggs, constantly make the entrance to their nests 
very narrow, and line the interior with an abundance 
of such materials as conduct heat slowly; while the 
Ring-Dove, which lays two eggs only, forms so slight 
a structure that they may frequently be seen through 
* T have pointed out the errors into which Dr. Darwin has 
fallen in his remarks on the Cuckoo in my observations on that 
bird, p. 72. 
