ost 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 or climbing, and most of the Fissirostral or
wide-gaped 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 undoubtedly very widely separated from the other
families with which they are associated in our classifications. The
Tits vary much in their mode of nesting, some making open nests
concealed in a hole, while others build domed or even pendulous 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 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 as the male, while the gorgeous
Chatterers, Manakins, Tanagers, and Birds of Paradise, as well as our
own Blackbird, have mates so dull and incon