A THEORY OF BIRDS’ NESTS. 233 
materials of their abodes, so as better to protect their 
young. The introduction of new enemies to eggs or 
young birds, might introduce many alterations tend- 
ing to their better concealment. A change in the 
vegetation of a country, would often necessitate the 
use of new materials. So, also, we may be sure, that 
as a species slowly became modified in any external or 
internal characters, it would necessarily change in some 
degree its mode of building. This effect would be 
produced by modifications of the most varied nature ; 
such as the power and rapidity of flight, which must 
often determine the distance to which a bird will go to 
obtain materials for its nest; the capacity of sustain- 
ing itself almost motionless in the air, which must 
sometimes determine the position in which a nest can 
be built; the strength and grasping power of the foot 
in relation to the weight of the bird, a power abso- 
lutely essential to the constructor of a delicately-woven 
and well-finished nest; the length and fineness of the 
beak, which has to be used like a needle in building 
the best textile nests; the length and mobility of the 
neck, which is needful for the same purpose ; the pos- 
session of a salivary secretion like that used in the 
nests of many of the swifts and swallows, as well as 
that of the song-thrush—peculiarities of habits, which 
ultimately depend on structure, and which often deter- 
mine the material most frequently met with or most 
easily to be obtained. Modifications in any of these 
characters would necessarily lead, either to a change in 
the materials of the nest, or in the mode of combin- 
