NESTS AND NEST-BUILDING IN BIRDS 181 



little or no knowledge of the conditions under which they were 

 produced, or of the variable factors indicated in the preceding 

 section. 



Aside from what may be regarded for the present as sporadic 

 or wholly exceptional cases, I think it can be shown that specific 

 variation of biological importance in nest-building is less than 

 has been supposed, and that much indiscriminate speculation 

 has arisen from confusing the non-essential with the more stable 

 and important characters. Nevertheless I recognize the diffi- 

 culties of the subject, and the present discussion is offered by 

 way of suggestion for future observations, in what seem to 

 me the right directions. Certainly the highly uniform and 

 stereotyped characters of the nests of birds, in relation to the 

 uniform and recurrent behavior of the builders, of which they 

 are the visible expression, is the most impressive lesson which 

 they teach, as it is the unimpeachable witness of the ruling 

 hand of instinct in all nest-building operations. 



Although there is no typical bird's nest in a strict sense, 

 there are specific types of nests, as well as general types, based 

 on typical activities of the builders. We shall consider the 

 general types first. A classification of the nest of birds upon 

 the basis of behavior is given in table 2, but that it is purely 

 tentative need hardly be emphasized. No classification in 

 sharply defined lines, is probably either possible or to be expected ; 

 moreover no classification can approach finality, until far more 

 is known than at present upon the whole subject of nidification, 

 and especially upon the behavior of the mated birds, and the 

 significance of variations in their products. 



The Caprimulgidae, of which the American i^ighthawk and 

 whippoorwill are typical, represent a considerable number of 

 birds which at the present day neither build nests of any sort, 

 nor adopt or adapt cavities of any description to conceal their 

 eggs. Yet it is interesting to notice that some of them become 

 as attached to their chosen brooding places, as martins or wood- 

 peckers to their adopted trees, and nighthawks, — presumably 

 the same individuals, — have been known to occupy the same 

 rock of even the same patch of ground for many successive 

 seasons. 



We regard the sand pile or mound of earth and leaves heaped 

 over its eggs by Maleo or true Megapode (No. i of table 2) as 



