A THEORY OF BIRDS’ NESTS, 239 
ciated 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. 
Seaual 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- 
