A THEORY OF BIRDS' NESTS. 239 



ciatecl in our classifications. The Tits vary much in 

 their mode of nesting, some making open nests con- 

 cealed in a hole, while others build domed or even pen- 

 dulous covered nests, but they all come under the same 

 class. Starlings vary in a similar way. The talking 

 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, 



Sexual differences of Colour in Birds. 



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 principle of sexual 

 selection. We can, to a great extent, understand 

 how male Pheasants and Grouse have acquired their 

 more brilliant plumage and greater size, by the con- 

 tinual 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, Parro- 



