CAUSES DETERMINING THE FORM. 113 



under cattle-sheds, against the sides of walls, and 

 in chimneys : that while some were not more than 

 three inches deep, others measured nearly nine: 

 while in some there was scarcely any grass, in 

 others it formed half of their nests. I have 

 observed the same nests of the cliff-swallow in 

 which the eggs had been deposited before the 

 pendent neck was added, and which remained so 

 until the birds had reared their brood, amidst 

 other nests furnished with a neck, which was much 

 larger in some than in others. From this I have 

 inferred that nests are formed more or less com- 

 pletely, in many instances, in accordance with the 

 necessity under which the bird may be of depo- 

 siting its eggs." * 



Accident often determines the form, the situ- 

 ation, and the materials of the nest ; and with the 

 variation in each, the art of its construction varies 

 also. Yet in many instances we become witnesses 

 of the same astonishing skill in the employment 

 of the materials, in the adaptation of the nest to 

 the situation or to the circumstances under which 

 it happens to be constructed. It has been well 

 said, " The nest of a bird is one of those daily 

 * Ornithological Biography. 

 I 



