A Theory of Birds' Nests 77 



shrikes, chatterers, and tanagers of the tropics, together with all 

 raptorial birds and pigeons, and a vast number ot others, in every 

 part of the world, all adopt the latter mode of building. 



It will be seen that this division of birds according to their 

 nidification, bears little relation to the character of the nest itself. 

 It is a functional not a structural classification. The most rude 

 and the most perfect specimens of bird-architecture are to be found 

 in both sections. It has, however, a certain relation to natural 

 affinities, for large groups of birds undoubtedly allied, fall into one 

 or the other division exclusively. The species of a genus or of a 

 family are rarely divided between the two primary classes, although 

 they are frequently divided between the two very distinct modes of 

 nidification that exist in the first of them. 



All the Scansorial, and most of the Fissirostral birds, for example, 

 build concealed nests ; and, in the latter group, the two families 

 which build open nests, the swifts and the goat-suckers, are un- 

 doubtedly very widely separated from the other families with which 

 they are associated in our classifications. The tits, also, vary much 

 in their mode of nesting : some making open nests concealed in a 

 hole, while others build domed or even pendulous covered nests, 

 so that they all come under the same class. Starlings vary in a 

 similar way. The Mynahs, like our own starlings, build in holes, 

 the glossy starlings of the East (of the genus Calornis) form a 

 hanging covered nest, while the genus Sturnopastor builds in a 

 hollow tree. One of the most striking cases, in which one family 

 of birds is divided between the two classes, is that of the finches ; 

 for, while most of the European species build exposed nests, many 

 of the Australian finches make them dome-shaped. 



Turning now from the nests to the creatures who make them, 

 let us consider birds themselves from a somewhat unusual point of 

 view, and form them into separate groups, according as both sexes, 

 or the males only, are adorned with conspicuous colours. 



The sexual differences of colour and plumage in birds are very 

 remarkable and have attracted much attention ; and in the case of 

 polygamous birds have been well explained by Mr Darwin's princi- 

 ple of sexual selection. We can, to some extent, understand how 

 male pheasants and grouse have acquired their more brilliant 

 plumage and greater size, by the continual rivalry of the males, both 

 in strength and beauty ; but this theory does not throw any light 

 on the causes which have made the female toucan, bee-eater, 

 parroquet, macaw and tit, in almost every case as gay and brilliant 



