190 A HISTORY OF BIRDS 



the same way as Sparrows, Starlings and Swifts build within the 

 roofs of human dwellings. 



In broad outline the main facts concerning the construction 

 of nests, and the sites in which they are placed, have now been 

 surveyed, and here and there exceptions to the rule have been 

 cited. To these exceptional cases it is proposed to add a few 

 others, inasmuch as while some yet seem inexplicable, others 

 throw some light at least on the question of the evolution of 

 nest-building. 



That some species are remarkably conservative in the con- 

 struction of their nests, and in the selection of its site, while 

 others are not a little erratic in these matters, is well known. 

 But most of these instances will be found on examination to 

 be determined by circumstances. Thus Rooks have been known 

 to build theirnests of wire, of which an abundant supply was to 

 be obtained from refuse heaps in the neighbourhood, while 

 sticks were probably scarce, possibly having been appropriated 

 by earlier builders. Robins and Blue Titmice choose most 

 curious places wherein to build ; and here again we have an 

 explanation in that both these birds seek the neighbourhood of 

 human dwellings during the nesting period, thereby displaying 

 a confidence that happily is not often abused. 



Similarly, the fact that Cormorants and Herons, and even 

 the lordly Eagle, will build either in trees, precipitous rocks, 

 or on the ground is explained by the nature of the environment. 

 Where lofty trees are available all seem to prefer the security 

 they offer. 



But the exceptions to which reference is here particularly 

 made are those which break with the traditions of the tribe in 

 a much more striking fashion, inasmuch as, in the case of large 

 groups of birds especially, some species are found which have 

 developed idiosyncrasies in this matter which seem to defy solu- 

 tion ; and while with some species these departures are sporadic, 

 it is often found that with others of the same group they have 

 become normal. Thus among the Duck tribe the ground is the 

 normal nesting-place, but the Mallard often elects to nest in a 

 hole in a tree thirty or forty feet from the ground ; with the 

 Golden-eye Duck (Clangula glaucion) such a site is invariably 

 chosen. 



Among the Plover tribe there is a remarkable lack of 



