102 
NATURAL SELECTION 
v 
form and structure of nests, that vary so much, and are so 
wonderfully adapted to the wants and habits of each species ; 
how are these to be accounted for except by instinct? I 
reply : They may be in a great measure explained by the 
general habits of the species, the nature of the tools they 
have to work with, and the materials they can most easily 
obtain, with the very simplest adaptations of means to an 
end, quite within the mental capacities of birds. The delicacy 
and perfection of the nest will bear a direct relation to the 
size of the bird, its structure and habits. That of the wren 
or the humming-bird is perhaps not finer or more beautiful in 
proportion than that of the blackbird, the magpie, or the 
crow. The wren, having a slender beak, long legs, and great 
activity, is able with great ease to form a well-woven nest of 
the finest materials, and places it in thickets and hedgerows 
which it frequents in its search for food. The titmouse, 
haunting fruit-trees and walls, and searching in cracks and 
crannies for insects, is naturally led to build in holes where it 
has shelter and security; while its great activity, and the 
perfection of its tools (bill and feet) enable it readily to form 
a beautiful receptacle for its eggs and young. Pigeons 
having heavy bodies and weak feet and bills (imperfect tools 
for forming a delicate structure) build rude, flat nests of 
sticks, laid across strong branches, which will bear their 
weight and that of their bulky young. They can do no 
better. The Caprimulgidse have the most imperfect tools of 
all, feet that will not support them except on a flat surface 
(for they cannot truly perch) and a bill excessively broad, 
short, and weak, and almost hidden by feathers and bristles. 
They cannot build a nest of twigs or fibres, hair or moss, like 
other birds, and they therefore generally dispense with one 
altogether, laying their eggs on the bare ground, or on the 
stump or flat limb of a tree. The clumsy hooked bills, short 
necks and feet, and heavy bodies of parrots, render them 
quite incapable of building a nest like most other birds. 
They cannot climb up a branch without using both bill and 
feet ; they cannot even turn round on a perch without holding 
on with their bill. How, then, could they inlay, or weave, 
or twist the materials of a nest ? Consequently they all lay 
in holes of trees, the tops of rotten stumps, or in deserted 
