vi A THEORY OF BIRDS’ NESTS 119 
Changed Conditions and persistent Habits as influencing 
Nidification 
Besides the causes above alluded to, there are two other 
factors whose effect in any particular case we can only vaguely 
guess at, but which must have had an important influence in 
determining the existing details of nidification. These are— 
changed conditions of existence, whether internal or external, 
and the influence of hereditary or imitative habit; the first 
inducing alterations in accordance with changes of organic 
structure, of climate, or of the surrounding fauna and flora ; 
the other preserving the peculiarities so produced, even when 
changed conditions render them no longer necessary. Many 
facts have been already given which show that birds do adapt 
their nests to the situations in which they place them, and the 
adoption of eaves, chimneys, and boxes by swallows, wrens, 
and many other birds, shows that they are always ready to 
take advantage of changed conditions. It is probable, there- 
fore, that a permanent change of climate would cause many 
birds to modify the form or materials of their abodes, so as 
better to protect their young. The introduction of new 
enemies to eggs or young birds might introduce many alter- 
ations tending 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 build- 
ing. 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 sustaining 
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 absolutely essential to the con- 
structor 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 possession of a salivary secretion like that used in the 
