brilliantly coloured in both sexes, and the nest is in a hole of a tree.
10. Ground cuckoos (Centropus). These birds are often of conspicuous
colours, and are alike in both sexes. They build a domed nest.
11. Woodpeckers (Picidae). In this family the females often differ from
the males, in having a yellow or white, instead of a crimson crest, but
are almost as conspicuous. They all nest in holes in trees.
12. Parrots (Psittaci). In this great tribe, adorned with the most
brilliant and varied colours, the rule is, that the sexes are precisely
alike, and this is the case in the most gorgeous families, the lories,
the cockatoos, and the macaws; but in some there is a sexual difference
of colour to a slight extent. All build in holes, mostly in trees, but
sometimes in the ground, or in white ants' nests. In the single case in
which the nest is exposed, that of the Australian ground parrot,
Pezoporus formosus, the bird has lost the gay colouring of its allies,
and is clothed in sombre and completely protective tints of dusky green
and black.
13. Gapers (Eurylaemidae). In these beautiful Eastern birds, somewhat
allied to the American chatterers, the sexes are exactly alike, and are
adorned with the most gay and conspicuous markings. The nest is a woven
structure, _covered over_, and suspended from the extremities of
branches over water.
14. Pardalotus (Ampelidae). In these Australian birds the females differ
from the males, but are often very conspicuous, having brightly-spotted
heads. Their nests are sometimes dome-shaped, sometimes in holes of
trees, or in burrows in the ground.
15. Tits (Paridae). These little birds are always pretty, and many
(especially among the Indian species) are very conspicuous. They always
have the sexes alike, a circumstance very unusual among the smaller
gaily-coloured birds of our own country. The nest is always covered over
or concealed in a hole.
16. Nuthatches (Sitta). Often very pretty birds, the sexes alike, and
the nest in a hole.
17.---- (Sittella). The female of these Australian nuthatches is often
the most conspicuous, being white-and black-marked. The nest is,
according to Gould, "completely concealed among upright twigs connected
together."
18. Creepers (Climacteris). In these Australian creepers the sexes are
alike, or the female most conspicuous, and the nest is in a hole of a
tree.
19. Estrelda, Amadina. In these genera of Eastern and Australian finches
the females, although more or less different from the males, are still
very conspicuous having a red rump, or being white spotted. They differ
from most others of the family in building domed nests.
20. Certhiola. In these pretty little American creepers the sexes are
alike, and they build a domed nest.
21. Mynahs (Sturnidae). These showy Eastern starlings have the sexes
exactly alike. They build in holes of trees.
22. Calornis (Sturnidae). These brilliant metallic starlings have no
sexual differences. They build a pensile covered nest.
23. Hangnests (Icteridae). The red or yellow and black plumage of most of
these birds is very conspicuous, and is exactly alike in both sexes.
They are celebrated for their fine purse-shaped pensile nests.
It will be seen that this list comprehends six important families of
Fissirostres, four of Scansores, the Psittaci, and several genera, with
three entire families of Passeres, comprising about twelve hundred
species, or about one-seventh of all known birds.
* * * * *
The cases in which, whenever the male is gaily coloured, the female is
much less gay or quite inconspicuous, are exceedingly numerous,
comprising, in fact, almost all the bright-coloured Passeres, except
those enumerated in the preceding class. The following are the most
remarkable:--
1. Chatterers (Cotingidae). These comprise some of the most gorgeous
birds in the world, vivid blues, rich purples, and bright reds, being
the most characteristic colours. The females are always obscurely
tinted, and are often of a greenish hue, not easily visible among the
foliage.
2. Manakins (Pipridae). These elegant birds, whose caps or crests are of
the most brilliant colours, are usually of a sombre green in the female
sex.
3. Tanagers (Tanagridae). These rival the chatterers in the brilliancy of
their colours, and are even more varied. The females are generally of
plain and sombre hues, and always less conspicuous than the males.
In the extensive families of the warblers (Sylviadae), thrushes
(Turdidae), flycatchers (Muscicapidae), and shrikes (Laniadae), a
considerable proportion of the species are beautifully marked with gay
and conspicuous tints, as is also the case in the Pheasants and Grouse;
but in every case the females are less gay, and are most frequently of
the very plainest and least conspicuous hues. Now, throughout _the whole
of these families the nest is open_, and I am not aware of a single
instance in which any one of these birds builds a _domed nest_, or
places it in a _hole of a tree_, or _underground_, or in any place where
it is effectually concealed.
In considering the question we are now investigating, it is not
necessary to take into account the larger and more powerful birds,
because these seldom depend much on concealment to secure their safety.
In the raptorial birds bright colours are as a rule absent; and their
structure and habits are such as not to require any special protection
for the female. The larger waders are sometimes very brightly coloured
in both sexes; but they are probably little subject to the attacks of
enemies, since the scarlet ibis, the most conspicuous of birds, exists
in immense quantities in South America. In game birds and water-fowl,
however, the females are often very plainly coloured, when the males are
adorned with brilliant hues; and the abnormal family of the Megapodidae
offers us the interesting fact of an identity in the colours of the
sexes (which in Megacephalon and Talegalla are somewhat conspicuous), in
conjunction with the habit of not sitting on the eggs at all.
_What the Facts Teach us._
Taking the whole body of evidence here brought forward, embracing as it
does almost every group of bright-coloured birds, it will, I think, be
admitted that the relation between the two series of facts in the
colouring and nidification of birds has been sufficiently established.
There are, it is true, a few apparent and some real exceptions, which I
shall consider presently; but they are too few and unimportant to weigh
much against the mass of evidence on the other side, and may for the
present be neglected. Let us then consider what we are to do with this
unexpected set of correspondences between groups of phenomena which, at
first sight, appear so disconnected. Do they fall in with any other
groups of natural phenomena? Do they teach us anything of the way in
which nature works, and give us any insight into the causes which have
brought about the marvellous variety, and beauty, and harmony of living
things? I believe we can answer these questions in the affirmative; and
I may mention, as a sufficient proof that these are not isolated facts,
that I was first led to see their relation to each other by the study of
an analogous though distinct set of phenomena among insects, that of
protective resemblance and "mimicry."
On considering this remarkable series of corresponding facts, the first
thing we are taught by them seems to be, that there is no incapacity in
the female sex among birds, to receive the same bright hues and strongly
contrasted tints with which their partners are so often decorated, since
whenever they are _protected and concealed_ during the period of
incubation _they are similarly adorned_. The fair inference is, that it
is chiefly due to the absence of protection or concealment during this
important epoch, that gay and conspicuous tints are withheld or left
undeveloped. The mode in which this has been effected is very
intelligible, if we admit the action of natural and sexual selection. It
would appear from the numerous cases in which both sexes are adorned
with equally brilliant colours (while both sexes are rarely armed with
equally developed offensive and defensive weapons when not required for
individual safety), that the normal action of "sexual selection" is to
develop colour and beauty in both sexes, by the preservation and
multiplication of all varieties of colour in either sex which are
pleasing to the other. Several very close observers of the habits of
animals have assured me, that male birds and quadrupeds do often take
very strong likes and dislikes to individual females, and we can hardly
believe that the one sex (the female) can have a general taste for
colour while the other has no such taste. However this may be, the fact
remains, that in a vast number of cases the female acquires as brilliant
and as varied colours as the male, and therefore most probably acquires
them in the same way as the male does; that is, either because the
colour is useful to it, or is correlated with some useful variation, or
is pleasing to the other sex. The only remaining supposition is that it
is transmitted from the other sex, without being of any use. From the
number of examples above adduced of bright colours in the female, this
would imply that colour-characters acquired by one sex are generally
(but not necessarily) transmitted to the other. If this be the case it
will, I think, enable us to explain the phenomena, even if we do not
admit that the male bird is ever influenced in the choice of a mate by
her more gay or perfect plumage.
The female bird, while sitting on her eggs in an uncovered nest, is much
exposed to the attacks of enemies, and any modification of colour which
rendered her more conspicuous would often lead to her destruction and
that of her offspring. All variations of colour in this direction in the
female, would therefore sooner or later be eliminated, while such
modifications as rendered her inconspicuous, by assimilating her to
surrounding objects, as the earth or the foliage, would, on the whole,
survive the longest, and thus lead to the attainment of those brown or
green and inconspicuous tints, which form the colouring (of the upper
surface at least), of the vast majority of female birds which sit upon
open nests.
This does not imply, as some have thought, that all female birds were
once as brilliant as the males. The change has been a very gradual one,
generally dating from the origin of genera or of larger groups, but
there can be no doubt that the remote ancestry of birds having great
sexual differences of colour, were nearly or quite alike, sometimes
(perhaps in most cases) more nearly resembling the female, but
occasionally perhaps being nearer what the male is now. The young birds
(which usually resemble the females) will probably give some idea of
this ancestral type, and it is well known that the young of allied
species and of different sexes are often undistinguishable.
_Colour more variable than Structure or Habits, and therefore the
Character which has generally been Modified._
At the commencement of this essay, I have endeavoured to prove, that the
characteristic differences and the essential features of birds' nests,
are dependent on the structure of the species and upon the present and
past conditions of their existence. Both these factors are more
important and less variable than colour; and we must therefore conclude
that in most cases the mode 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.
_Exceptional Cases confirmatory of the above Explanation._
There exist a few very curious and anomalous facts in the natural
history of birds, which fortunately serve as crucial tests of the truth
of this mode of explaining the inequalities of sexual colouration. It
has been long known, that in some species the males either assisted in,
or wholly performed, the act of incubation. It has also been often
noticed, that in certain birds the usual sexual differences were
reversed, the male being the more plainly coloured, the female more gay
and often larger. I am not, however, aware that these two anomalies had
ever been supposed to stand to each other in the relation of cause and
effect, till I adduced them in support of my views of the general theory
of protective adaptation. Yet it is undoubtedly the fact, that in the
best known cases in which the female bird is more conspicuously coloured
than the male, it is either positively ascertained that the latter
performs the duties of incubation, or there are good reasons for
believing such to be the case. The most satisfactory example is that of
the Gray Phalarope (Phalaropus fulicarius), the sexes of which are alike
in winter, while in summer the female instead of the male takes on a gay
and conspicuous nuptial plumage; but the male performs the duties of
incubation, sitting upon the eggs, which are laid upon the bare ground.
In the Dotterell (Eudromias morinellus) the female is larger and more
brightly coloured than the male; and here, also, it is almost certain
that the latter sits upon the eggs. The Turnices of India also, have the
female larger and often more brightly coloured; and Mr. Jerdon states,
in his "Birds of India," that the natives report, that, during the
breeding season, the females desert their eggs and associate in flocks,
while the males are employed in hatching the eggs. In the few other
cases in which the females are more brightly coloured, the habits are
not accurately known. The case of the Ostriches and Emeus will occur to
many as a difficulty, for here the male incubates, but is not less
conspicuous than the female; but there are two reasons why the case does
not apply;--the birds are too large to derive any safety from
concealment, from enemies which would devour the eggs they can defend
themselves by force, while to escape from their personal foes they trust
to speed.
We find, therefore, that a very large mass of facts relating to the
sexual colouration and the mode of nidification of birds, including some
of the most extraordinary anomalies to be found in their natural
history, can be shown to have an interdependent relation to each other,
on the simple principle of the need of greater protection to that parent
which performs the duties of incubation. Considering the very imperfect
knowledge we possess of the habits of most extra-European birds, the
exceptions to the prevalent rule are few, and generally occur in
isolated species or in small groups; while several apparent exceptions
can be shown to be really confirmations of the law.
_Real or apparent Exceptions to the Law stated at page 240._
The only marked exceptions I have been able to discover are the
following:--
1. King crows (Dicrourus). These birds are of a glossy black colour with
long forked tails. The sexes present no difference, and they build open
nests. This apparent exception may probably be accounted for by the fact
that these birds do not need the protection of a less conspicuous
colour. They are very pugnacious, and often attack and drive away crows,
hawks, and kites; and as they are semi-gregarious in their habits, the
females are not likely to be attacked while incubating.
2. Orioles (Oriolidae). The true orioles are very gay birds; the sexes
are, in many Eastern species, either nearly or quite alike, and the
nests are open. This is one of the most serious exceptions, but it is
one that to some extent proves the rule; for in this case it has been
noticed, that the parent birds display excessive care and solicitude in
concealing the nest among thick foliage, and in protecting their
offspring by incessant and anxious watching. This indicates that the
want of protection consequent on the bright colour of the female makes
itself felt, and is obviated by an increased development of the mental
faculties.
3. Ground thrushes (Pittidae). These elegant and brilliantly-coloured
birds are generally alike in both sexes, and build an open nest. It is
curious, however, that this is only an apparent exception, for almost
all the bright colours are on the under surface, the back being usually
olive green or brown, and the head black, with brown or whitish stripes,
all which colours would harmonize with the foliage, sticks, and roots
which surround the nest, built on or near the ground, and thus serve as
a protection to the female bird.
4. Grallina Australis. This Australian bird is of strongly contrasted
black and white colours. The sexes are exactly alike, and it builds an
open clay nest in an exposed situation on a tree. This appears to be a
most striking exception, but I am by no means sure that it is so. We
require to know what tree it usually builds on, the colour of the bark
or of the lichens that grow upon it, the tints of the ground, or of
other surrounding objects, before we can say that the bird, when sitting
on its nest, is really conspicuous. It has been remarked that small
patches of white and black blend at a short distance to form grey, one
of the commonest tints of natural objects.
5. Sunbirds (Nectarineidae). In these beautiful little birds the males
only are adorned with brilliant colours, the females being quite plain,
yet they build covered nests in all the cases in which the nidification
is known. This is a negative rather than a positive exception to the
rule, since there may be other causes besides the need for protection,
which prevent the female acquiring the gay colours of her mate, and
there is one curious circumstance which tends to elucidate it. The male
of Leptocoma zeylanica is said to assist in incubation. It is possible,
therefore, that the group may originally have used open nests, and some
change of conditions, leading the male bird to sit, may have been
followed by the adoption of a domed nest. This is, however, the most
serious exception I have yet found to the general rule.
6. Superb warblers (Maluridae). The males of these little birds are
adorned with the most gorgeous colours, while the females are very
plain, yet they make domed nests. It is to be observed, however, that
the male plumage is nuptial merely, and is retained for a very short
time; the rest of the year both sexes are plain alike. It is probable,
therefore, that the domed nest is for the protection of these delicate
little birds against the rain, and that there is some unknown cause
which has led to the development of colour in the males only.
There is one other case which at first sight looks like an exception,
but which is far from being one in reality, and deserves to be
mentioned. In the beautiful Waxwing, (Bombycilla garrula,) the sexes are
very nearly alike, and the elegant red wax tips to the wing-feathers are
nearly, and sometimes quite, as conspicuous in the female as in the
male. Yet it builds an open nest, and a person looking at the bird would
say it ought according to my theory to cover its nest. But it is, in
reality, as completely protected by its colouration as the most plainly
coloured bird that flies. It breeds only in very high latitudes, and the
nest, placed in fir-trees, is formed chiefly of lichens. Now the
delicate gray and ashy and purplish hues of the head and back, together
with the yellow of the wings and tail, are tints that exactly harmonize
with the colours of various species of lichens, while the brilliant red
wax tips exactly represent the crimson fructification of the common
lichen, Cladonia coccifera. When sitting on its nest, therefore, the
female bird will exhibit no colours that are not common to the materials
of which it is constructed; and the several tints are distributed in
about the same proportions as they occur in nature. At a short distance
the bird would be indistinguishable from the nest it is sitting on, or
from a natural clump of lichens, and will thus be completely protected.
I think I have now noticed all exceptions of any importance to the law
of dependence of sexual colour on nidification. It will be seen that
they are very few in number, compared with those which support the
generalization; and in several cases there are circumstances in the
habits or structure of the species that sufficiently explain them. It is
remarkable also that I have found scarcely any _positive_ exceptions,
that is, cases of very brilliant or conspicuous female birds in which
the nest was not concealed. Much less can there be shown any group of
birds, in which the females are all of decidedly conspicuous colours on
the upper surface, and yet sit in open nests. The many cases in which
birds of dull colours in both sexes make domed or concealed nests, do
not, of course, affect this theory one way or the other; since its
purpose is only to account for the fact, that brilliant females of
brilliant males are _always_ found to have covered or hidden nests,
while obscure females of brilliant males _almost always_ have open and
exposed nests. The fact that all classes of nests occur with dull
coloured birds in both sexes merely shows, as I have strongly
maintained, that in most cases the character of the nest determines the
colouration of the female, and not _vice versa_.
If the views here advocated are correct, as to the various influences
that have determined the specialities of every bird's nest, and the
general colouration of female birds, with their action and reaction on
each other, we can hardly expect to find evidence more complete than
that here set forth. Nature is such a tangled web of complex relations,
that a series of correspondences running through hundreds of species,
genera, and families, in every part of the system, can hardly fail to
indicate a true casual connexion; and when, of the two factors in the
problem, one can be shown to be dependent on the most deeply seated and
the most stable facts of structure and conditions of life, while the
other is a character universally admitted to be superficial and easily
modified, there can be little doubt as to which is cause and which
effect.
_Various modes of Protection of Animals._
But the explanation of the phenomenon here attempted does not rest alone
on the facts I have been able now to adduce. In the essay on "Mimicry,"
it is shown how important a part the necessity for protection has
played, in determining the external form and colouration, and sometimes
even the internal structure of animals.