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 



