re lying down, believed to be that of the wild ancestral form
which thus smoothed down the herbage so as to form a comfortable bed. We
see, too, that the general mode of nesting is characteristic of whole
families differing widely in size, form, and colours. Thus, all the
kingfishers and their allies in every part of the world nest in holes,
usually in banks, but sometimes in trees. The motmots and the puff-birds
(Bucconidae) build in similar places; while the toucans, barbets,
trogons, woodpeckers, and parrots all make their nests in hollow trees.
This habit, pervading all the members of extensive families, must
therefore be extremely ancient, more especially as it evidently depends
in some degree on the structure of the birds, the bills, and especially
the feet, of all these groups being unfitted for the construction of
woven arboreal nests.[123] But in all these families the colour varies
greatly from species to species, being constant only in the one
character of the similarity of the sexes, or, at all events, in their
being equally conspicuous even though differently coloured.

When I first put forward this view of the connection between the mode of
nesting and the coloration of female birds, I expressed the law in
somewhat different terms, which gave rise to some misunderstanding, and
led to numerous criticisms and objections. Several cases were brought
forward in which the females were far less brilliant than the males,
although the nest was covered. This is the case with the Maluridae, or
superb warblers of Australia, in which the males are very brilliant
during the pairing season and the females quite plain, yet they build
domed nests. Here, there can be little doubt, the covered nest is a
protection from rain or from some special enemies to the eggs; while the
birds themselves are protectively coloured in both sexes, except for a
short time during the breeding season when the male acquires brilliant
colours; and this is probably connected with the fact of their
inhabiting the open plains and thin scrub of Australia, where protective
colours are as generally advantageous as they are in our north-temperate
zones.

As I have now stated the law, I do not think there are any exceptions to
it, while there are an overwhelming number of cases which give it a
strong support. It has been objected that the domed nests of many birds
are as conspicuous as the birds themselves would be, and would,
therefore, be of no use a