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- 



