Contributions to the theory of natural selection. A series of essays ... By Alfred Russel Wallace ...

250 A THEORY OF BIRDS' NESTS.
of nidification (dependent on structure and environment) has been the cause, and not the effect, of the
similarity or differences of the sexes as regards colour.
When the confirmed habit of a group of birds, was to
build their nests in holes of trees like the toucans, or in
holes in the ground like the kingfishers, the protection
the female thus obtained, during the important and
dangerous time of incubation, placed the two sexes on
an equality as regards exposure to attack, and allowed
" sexual selection," or any other cause, to act unchecked
in the development of gay colours and conspicuous
markings in both sexes.
When, on the other hand (as in the Tanagers and
Flycatchers), the habit of the whole group was to build
open cup-shaped nests in more or less exposed situations, the production of colour and marking in the
female, by whatever cause, was continually checked by
its rendering her too conspicuous, while in the male it
had free play, and developed in him the most gorgeous
hues. This, however, was not perhaps universally the
case; for where there was more than usual intelligence
and capacity for change of habits, the danger the female was exposed to by a partial brightness of colour
or marking might lead to the construction of a concealed
or covered nest, as in the case of the Tits and Hangnests. When this occurred, a special protection to the
female would be no longer necessary; so that the
acquisition of colour and the modification of the nest,
might in some cases act and react on each other and
attain their full development together.