WE have in this chapter to consider
why the females of many birds
have not acquired the same ornaments
as the male; and why, on the
other hand, both sexes of many
other birds are equally, or almost
equally, ornamented? In the following
chapter we shall consider the
few cases in which the female
is more conspicuously coloured
than
the male.
In my Origin of Species* I briefly
suggested that the long tail
of the peacock would be inconvenient
and the conspicuous black colour
of the male capercailzie dangerous,
to the female during the period
of incubation: and consequently
that the transmission of these
characters from the male to the
female offspring had been checked
through natural selection. I
still think that this may have
occurred in some few instances:
but after mature reflection on
all the facts which I have been
able to collect, I am now inclined
to believe that when the sexes
differ, the successive variations
have generally been from the
first limited in their transmission
to the same sex in which they
first arose. Since my remarks
appeared, the subject of sexual
colouration has been discussed
in some very interesting papers
by Mr. Wallace,*(2) who believes
that in almost all cases the
successive variations tended
at first to be transmitted equally
to both sexes; but that the female
was saved, through natural selection,
from acquiring the conspicuous
colours of the male, owing to
the danger which she would thus
have incurred during incubation.
* Fourth edition, 1866, p. 241.
*(2) Westminster Review, July,
1867. Journal of Travel, vol.
i., 1868, p. 73.
This view necessitates
a tedious discussion on a difficult
point,
namely, whether the transmission
of a character, which is at first
inherited by both sexes can be
subsequently limited in its transmission
to one sex alone by means of
natural selection. We must bear
in mind, as shewn in the preliminary
chapter on sexual selection,
that characters which are limited
in their development to one sex
are always latent in the other.
An imaginary illustration will
best aid us in seeing the difficulty
of the case; we may suppose that
a fancier wished to make a breed
of pigeons, in which the males
alone should be coloured of a
pale blue, whilst the females
retained their former slaty tint.
As with pigeons characters of
all kinds are usually transmitted
to both sexes equally, the fancier
would have to try to convert
this latter form of inheritance
into sexually-limited transmission.
All that he could do would be
to persevere in selecting every
male pigeon which was in the
least degree of a paler blue;
and the natural result of this
process, if steadily carried
on for a long time, and if the
pale variations were strongly
inherited or often recurred,
would be to make his whole stock
of a lighter blue. But our fancier
would be compelled to match,
generation after generation,
his pale blue males with slaty
females, for he wishes to keep
the latter of this colour. The
result would generally be the
production either of a mongrel
piebald lot, or more probably
the speedy and complete loss
of the pale-blue tint; for the
primordial slaty colour would
be transmitted with prepotent
force. Supposing, however, that
some pale-blue males and slaty
females were produced during
each successive generation, and
were always crossed together,
then the slaty females would
have, if I may use the expression,
much blue blood in their veins,
for their fathers, grandfathers, &c.,
will all have been blue birds.
Under these circumstances it
is conceivable (though I know
of no distinct facts rendering
it probable) that the slaty females
might acquire so strong a latent
tendency to pale-blueness, that
they would not destroy this colour
in their male offspring, their
female offspring still inheriting
the slaty tint. If so, the desired
end of making a breed with the
two sexes permanently different
in colour might be gained.
The extreme importance, or rather
necessity in the above case of
the desired character, namely,
pale-blueness, being present
though in a latent state in the
female, so that the male offspring
should not be deteriorated, will
be best appreciated as follows:
the male of Soemmerring's pheasant
has a tail thirty-seven inches
in length, whilst that of the
female is only eight inches;
the tail of the male common pheasant
is about twenty inches, and that
of the female twelve inches long.
Now if the female Soemmerring
pheasant with her short tail
were crossed with the male common
pheasant, there can be no doubt
that the male hybrid offspring
would have a much longer tail
than that of the pure offspring
of the common pheasant. On the
other hand, if the female common
pheasant, with a tail much longer
than that of the female Soemmerring
pheasant, were crossed with the
male of the latter, the male
hybrid offspring would have a
much shorter tail than that of
the pure offspring of Soemmerring's
pheasant.*
* Temminck says that the tail
of the female Phasianus Soemmerringii
is only six inches long, Planches
coloriees, vol. v., 1838, pp.
487 and 488: the measurements
above given were made for me
by Mr. Sclater. For the common
pheasant, see Macgillivray, History
of British Birds, vol. i., pp.
118-121.
Our fancier, in order to make
his new breed with the males
of a pale-blue tint, and the
females unchanged, would have
to continue selecting the males
during many generations; and
each stage of paleness would
have to be fixed in the males,
and rendered latent in the females.
The task would be an extremely
difficult one, and has never
been tried, but might possibly
be successfully carried out.
The chief obstacle would be the
early and complete loss of the
pale-blue tint, from the necessity
of reiterated crosses with the
slaty female, the latter not
having at first any latent tendency
to produce pale-blue offspring.
On the other hand, if one or
two males were to vary ever so
slightly in paleness, and the
variations were from the first
limited in their transmission
to the male sex, the task of
making a new breed of the desired
kind would be easy, for such
males would simply have to be
selected and matched with ordinary
females. An analogous case has
actually occurred, for there
are breeds of the pigeon in Belgium*
in which the males alone are
marked with black striae. So
again Mr. Tegetmeier has recently
shewn*(2) that dragons not rarely
produce silver-coloured birds,
which are almost always hens;
and he himself has bred ten such
females. It is on the other hand
a very unusual event when a silver
male is produced; so that nothing
would be easier, if desired,
than to make a breed of dragons
with blue males and silver females.
This tendency is indeed so strong
that when Mr. Tegetmeier at last
got a silver male and matched
him with one of the silver females,
he expected to get a breed with
both sexes thus coloured; he
was however disappointed, for
the young male reverted to the
blue colour of his grandfather,
the young female alone being
silver. No doubt with patience
this tendency to reversion in
the males, reared from an occasional
silver male matched with a silver
hen, might be eliminated, and
then both sexes would be coloured
alike; and this very process
has been followed with success
by Mr. Esquilant in the case
of silver turbits.
* Dr. Chapius, Le Pigeon Voyageur
Belge, 1865, p. 87.
*(2) The Field, Sept., 1872.
With fowls, variations of colour,
limited in their transmission
to the male sex, habitually occur.
When this form of inheritance
prevails, it might well happen
that some of the successive variations
would be transferred to the female,
who would then slightly resemble
the male, as actually occurs
in some breeds. Or again, the
greater number, but not all,
of the successive steps might
be transferred to both sexes,
and the female would then closely
resemble the male. There can
hardly be a doubt that this is
the cause of the male pouter
pigeon having a somewhat larger
crop, and of the male carrier
pigeon having somewhat larger
wattles, than their respective
females; for fanciers have not
selected one sex more than the
other, and have had no wish that
these characters should be more
strongly displayed in the male
than in the female, yet this
is the case with both breeds.
The same process would have
to be followed, and the same
difficulties encountered, if
it were desired to make a breed
with the females alone of some
new colour.
Lastly, our fancier might wish
to make a breed with the two
sexes differing from each other,
and both from the parent species.
Here the difficulty would be
extreme, unless the successive
variations were from the first
sexually limited on both sides,
and then there would be no difficulty.
We see this with the fowl; thus
the two sexes of the pencilled
Hamburghs differ greatly from
each other, and from the two
sexes of the aboriginal Gallus
bankiva; and both are now kept
constant to their standard of
excellence by continued selection,
which would be impossible unless
the distinctive characters of
both were limited in their transmission.
The Spanish fowl offers a more
curious case; the male has an
immense comb, but some of the
successive variations, by the
accumulation of which it was
acquired, appear to have been
transferred to the female; for
she has a comb many times larger
than that of the females of the
parent species. But the comb
of the female differs in one
respect from that of the male,
for it is apt to lop over; and
within a recent period it has
been ordered by the fancy that
this should always be the case,
and success has quickly followed
the order. Now the lopping of
the comb must be sexually limited
in its transmission, otherwise
it would prevent the comb of
the male from being perfectly
upright, which would be abhorrent
to every fancier. On the other
hand, the uprightness of the
comb in the male must likewise
be a sexually-limited character,
otherwise it would prevent the
comb of the female from lopping
over.
From the foregoing illustrations,
we see that even with almost
unlimited time at command, it
would be an extremely difficult
and complex, perhaps an impossible
process, to change one form of
transmission into the other through
selection. Therefore, without
distinct evidence in each case,
I am unwilling to admit that
this has been effected in natural
species. On the other hand, by
means of successive variations,
which were from the first sexually
limited in their transmission,
there would not be the least
difficulty in rendering a male
bird widely different in colour
or in any other character from
the female; the latter being
left unaltered, or slightly altered,
or specially modified for the
sake of protection.
As bright colours are of service
to the males in their rivalry
with other males, such colours
would be selected whether or
not they were transmitted exclusively
to the same sex. Consequently
the females might be expected
often to partake of the brightness
of the males to a greater or
less degree; and this occurs
with a host of species. If all
the successive variations were
transmitted equally to both sexes,
the females would be indistinguishable
from the males; and this likewise
occurs with many birds. If, however,
dull colours were of high importance
for the safety of the female
during incubation, as with many
ground birds, the females which
varied in brightness, or which
received through inheritance
from the males any marked accession
of brightness, would sooner or
later be destroyed. But the tendency
in the males to continue for
an indefinite period transmitting
to their female offspring their
own brightness, would have to
be eliminated by a change in
the form of inheritance; and
this, as shewn by our previous
illustration, would be extremely
difficult. The more probable
result of the long-continued
destruction of the more brightly-coloured
females, supposing the equal
form of transmission to prevail
would be the lessening or annihilation
of the bright colours of the
males, owing to their continual
crossing with the duller females.
It would be tedious to follow
out all the other possible results;
but I may remind the reader that
if sexually limited variations
in brightness occurred in the
females, even if they were not
in the least injurious to them
and consequently were not eliminated,
yet they would not be favoured
or selected, for the male usually
accepts any female, and does
not select the more attractive
individuals; consequently these
variations would be liable to
be lost, and would have little
influence on the character of
the race; and this will aid in
accounting for the females being
commonly duller-coloured than
the males.
In the eighth chapter instances
were given, to which many might
here be added, of variations
occurring at various ages, and
inherited at the corresponding
age. It was also shewn that variations
which occur late in life are
commonly transmitted to the same
sex in which they first appear;
whilst variations occurring early
in life are apt to be transmitted
to both sexes; not that all the
cases of sexually-limited transmission
can thus be accounted for. It
was further shewn that if a male
bird varied by becoming brighter
whilst young, such variations
would be of no service until
the age for reproduction had
arrived, and there was competition
between rival males. But in the
case of birds living on the ground
and commonly in need of the protection
of dull colours, bright tints
would be far more dangerous to
the young and inexperienced than
to the adult males. Consequently
the males which varied in brightness
whilst young would suffer much
destruction and be eliminated
through natural selection; on
the other hand, the males which
varied in this manner when nearly
mature, notwithstanding that
they were exposed to some additional
danger, might survive, and from
being favoured through sexual
selection, would procreate their
kind. As a relation often exists
between the period of variation
and the form of transmission,
if the bright-coloured young
males were destroyed and the
mature ones were successful in
their courtship, the males alone
would acquire brilliant colours
and would transmit them exclusively
to their male offspring. But
I by no means wish to maintain
that the influence of age on
the form of transmission, is
the sole cause of the great difference
in brilliancy between the sexes
of many birds.
When the sexes of birds differ
in colour, it is interesting
to determine whether the males
alone have been modified by sexual
selection, the females having
been left unchanged, or only
partially and indirectly thus
changed; or whether the females
have been specially modified
through natural selection for
the sake of protection. I will
therefore discuss this question
at some length, even more fully
than its intrinsic importance
deserves; for various curious
collateral points may thus be
conveniently considered.
Before we enter on the subject
of colour, more especially in
reference to Mr. Wallace's conclusions,
it may be useful to discuss some
other sexual differences under
a similar point of view. A breed
of fowls formerly existed in
Germany* in which the hens were
furnished with spurs; they were
good layers, but they so greatly
disturbed their nests with their
spurs that they could not be
allowed to sit on their own eggs.
Hence at one time it appeared
to me probable that with the
females of the wild Gallinaceae
the development of spurs had
been checked through natural
selection, from the injury thus
caused to their nests. This seemed
all the more probable, as wing-spurs,
which would not be injurious
during incubation, are often
as well developed in the female
as in the male; though in not
a few cases they are rather larger
in the male. When the male is
furnished with leg-spurs the
female almost always exhibits
rudiments of them,- the rudiment
sometimes consisting of a mere
scale, as in Gallus. Hence it
might be argued that the females
had aboriginally been furnished
with well-developed spurs, but
that these had subsequently been
lost through disuse or natural
selection. But if this view be
admitted, it would have to be
extended to innumerable other
cases; and it implies that the
female progenitors of the existing
spur-bearing species were once
encumbered with an injurious
appendage.
* Bechstein, Naturgeschichte
Deutschlands, 1793, B. iii.,
339.
In some few genera and species,
as in Galloperdix, Acomus, and
the Javan peacock (Pavo muticus),
the females, as well as the males,
possess well-developed leg-spurs.
Are we to infer from this fact
that they construct a different
sort of nest from that made by
their nearest allies, and not
liable to be injured by their
spurs; so that the spurs have
not been removed? Or are we to
suppose that the females of these
several species especially require
spurs for their defence? It is
a more probable conclusion that
both the presence and absence
of spurs in the females result
from different laws of inheritance
having prevailed, independently
of natural selection. With the
many females in which spurs appear
as rudiments, we may conclude
that some few of the successive
variations, through which they
were developed in the males,
occurred very early in life,
and were consequently transferred
to the females. In the other
and much rarer cases, in which
the females possess fully developed
spurs, we may conclude that all
the successive variations were
transferred to them; and that
they gradually acquired and inherited
the habit of not disturbing their
nests.
The vocal organs and the feathers
variously modified for producing
sound, as well as the proper
instincts for using them, often
differ in the two sexes, but
are sometimes the same in both.
Can such differences be accounted
for by the males having acquired
these organs and instincts, whilst
the females have been saved from
inheriting them, on account of
the danger to which they would
have been exposed by attracting
the attention of birds or beasts
of prey? This does not seem to
me probable, when we think of
the multitude of birds which
with impunity gladden the country
with their voices during the
spring.* It is a safer conclusion
that, as vocal and instrumental
organs are of special service
only to the males during their
courtship, these organs were
developed through sexual selection
and their constant use in that
sex alone- the successive variations
and the effects of use having
been from the first more or less
limited in transmission to the
male offspring.
* Daines Barrington, however,
thought it probable (Philosophical
Transactions, 1773, p. 164) that
few female birds sing, because
the talent would have been dangerous
to them during incubation. He
adds, that a similar view may
possibly account for the inferiority
of the female to the male in
plumage.
Many analogous
cases could be adduced; those
for instance of
the plumes on the head being
generally longer in the male
than in the female, sometimes
of equal length in both sexes,
and occasionally absent in the
female,- these several cases
occurring in the same group of
birds. It would be difficult
to account for such a difference
between the sexes by the female
having been benefited by possessing
a slightly shorter crest than
the male, and its consequent
diminution or complete suppression
through natural selection. But
I will take a more favourable
case, namely the length of the
tail. The long train of the peacock
would have been not only inconvenient
but dangerous to the peahen during
the period of incubation and
whilst accompanying her young.
Hence there is not the least
a priori improbability in the
development of her tail having
been checked through natural
selection. But the females of
various pheasants, which apparently
are exposed on their open nests
to as much danger as the peahen,
have tails of considerable length.
The females as well as the males
of the Menura superba have long
tails, and they build a domed
nest, which is a great anomaly
in so large a bird. Naturalists
have wondered how the female
Menura could manage her tail
during incubation; but it is
now known* that she "enters the
nest head first, and then turns
round with her tail sometimes
over her back, but more often
bent round by her side. Thus
in time the tail becomes quite
askew, and is a tolerable guide
to the length of time the bird
has been sitting." Both sexes
of an Australian kingfisher (Tanysiptera
sylvia) have the middle tail-feathers
greatly lengthened, and the female
makes her nest in a hole; and
as I am informed by Mr. R. B.
Sharpe these feathers become
much crumpled during incubation.
* Mr. Ramsay, in Proc. Zoolog.
Soc., 1868, p. 50.
In these two latter cases the
great length of the tail-feathers
must be in some degree inconvenient
to the female; and as in both
species the tail-feathers of
the female are somewhat shorter
than those of the male, it might
be argued that their full development
had been prevented through natural
selection. But if the development
of the tail of the peahen had
been checked only when it became
inconveniently or dangerously
great, she would have retained
a much longer tail than she actually
possesses; for her tail is not
nearly so long, relatively to
the size of her body, as that
of many female pheasants, nor
longer than that of the female
turkey. It must also be borne
in mind that, in accordance with
this view, as soon as the tail
of the peahen became dangerously
long, and its development was
consequently checked, she would
have continually reacted on her
male progeny, and thus have prevented
the peacock from acquiring his
present magnificent train. We
may therefore infer that the
length of the tail in the peacock
and its shortness in the peahen
are the result of the requisite
variations in the male having
been from the first transmitted
to the male offspring alone.
We are led to a nearly similar
conclusion with respect to the
length of the tail in the various
species of pheasants. In the
Eared pheasant (Crossoptilon
auritum) the tail is of equal
length in both sexes, namely
sixteen or seventeen inches;
in the common pheasant it is
about twenty inches long in the
male and twelve in the female;
in Soemmerring's pheasant, thirty-seven
inches in the male and only eight
in the female; and lastly in
Reeve's pheasant it is sometimes
actually seventy-two inches long
in the male and sixteen in the
female. Thus in the several species,
the tail of the female differs
much in length, irrespectively
of that of the male; and this
can be accounted for, as it seems
to me, with much more probability,
by the laws of inheritance,-
that is by the successive variations
having been from the first more
or less closely limited in their
transmission to the male sex
than by the agency of natural
selection, resulting from the
length of tail being more or
less injurious to the females
of these several allied species.
We may now consider Mr. Wallace's
arguments in regard to the sexual
colouration of birds. He believes
that the bright tints originally
acquired through sexual selection
by the males would in all, or
almost all cases, have been transmitted
to the females, unless the transference
had been checked through natural
selection. I may here remind
the reader that various facts
opposed to this view have already
been given under reptiles, amphibians,
fishes and lepidoptera. Mr. Wallace
rests his belief chiefly, but
not exclusively, as we shall
see in the next chapter, on the
following statement,* that when
both sexes are coloured in a
very conspicuous manner, the
nest is of such a nature as to
conceal the sitting bird; but
when there is a marked contrast
of colour between the sexes,
the male being gay and the female
dull-coloured, the nest is open
and exposes the sitting bird
to view. This coincidence, as
far as it goes, certainly seems
to favour the belief that the
females which sit on open nests
have been specially modified
for the sake of protection; but
we shall presently see that there
is another and more probable
explanation, namely, that conspicuous
females have acquired the instinct
of building domed nests oftener
than dull-coloured birds. Mr.
Wallace admits that there are,
as might have been expected,
some exceptions to his two rules,
but it is a question whether
the exceptions are not so numerous
as seriously to invalidate them.
* Journal of Travel, edited
by A. Murray, vol. i., 1868,
p. 78.
There is in the first place
much truth in the Duke of Argyll's
remark* that a large domed nest
is more conspicuous to an enemy,
especially to all tree-haunting
carnivorous animals, than a smaller
open nest. Nor must we forget
that with many birds which build
open nests, the male sits on
the eggs and aids the female
in feeding the young: this is
the case, for instance, with
Pyranga aestiva,*(2) one of the
most splendid birds in the United
States, the male being vermilion,
and the female light brownish-green.
Now if brilliant colours had
been extremely dangerous to birds
whilst sitting on their open
nests, the males in these cases
would have suffered greatly.
It might, however, be of such
paramount importance to the male
to be brilliantly coloured, in
order to beat his rivals, that
this may have more than compensated
some additional danger.
* Journal of Travel, edited
by A. Murray, vol. i., 1868,
p. 281.
*(2) Audubon, Ornithological
Biography, vol. i., p. 233.
Mr. Wallace admits that with
the king-crows (Dicrurus), orioles,
and Pittidae, the females are
conspicuously coloured, yet build
open nests; but he urges that
the birds of the first group
are highly pugnacious and could
defend themselves; that those
of the second group take extreme
care in concealing their open
nests, but this does not invariably
hold good;* and that with the
birds of the third group the
females are brightly coloured
chiefly on the under surface.
Besides these cases, pigeons
which are sometimes brightly,
and almost always conspicuously
coloured, and which are notoriously
liable to the attacks of birds
of prey, offer a serious exception
to the rule, for they almost
always build open and exposed
nests. In another large family,
that of the humming-birds, all
the species build open nests,
yet with some of the most gorgeous
species the sexes are alike;
and in the majority, the females,
though less brilliant than the
males, are brightly coloured.
Nor can it be maintained that
all female humming-birds, which
are brightly coloured, escape
detection by their tints being
green, for some display on their
upper surfaces red, blue, and
other colours.*(2)
* Jerdon, Birds of India, vol.
ii., p. 108. Gould's Handbook
of the Birds of Australia, vol.
i., p. 463.
*(2) For instance, the female
Eupetomena macroura has the head
and tail dark blue with reddish
loins; the female Lampornis porphyrurus
is blackish-green on the upper
surface, with the lores and sides
of the throat crimson; the female
Eulampis jugularis has the top
of the head and back green, but
the loins and the tail are crimson.
Many other instances of highly
conspicuous females could be
given. See Mr. Gould's magnificent
work on this family.
In regard to birds which build
in holes or construct domed nests,
other advantages, as Mr. Wallace
remarks, besides concealment
are gained, such as shelter from
the rain, greater warmth, and
in hot countries protection from
the sun;* so that it is no valid
objection to his view that many
birds having both sexes obscurely
coloured build concealed nests.*(2)
The female horn-bill (Buceros),
for instance, of India and Africa
is protected during incubation
with extraordinary care, for
she plasters up with her own
excrement the orifice of the
hole in which she sits on her
eggs, leaving only a small orifice
through which the male feeds
her; she is thus kept a close
prisoner during the whole period
of incubation;*(3) yet female
horn-bills are not more conspicuously
coloured than many other birds
of equal size which build open
nests. It is a more serious objection
to Mr. Wallace's view, as is
admitted by him, that in some
few groups the males are brilliantly
coloured and the females obscure,
and yet the latter hatch their
eggs in domed nests. This is
the case with the Grallinae of
Australia, the superb warblers
(Maluridae) of the same country,
the sun-birds (Nectariniae),
and with several of the Australian
honey-suckers or Meliphagidae.*(4)
* Mr. Salvin noticed in Guatemala
(Ibis, 1864, p. 375) that humming-birds
were much more unwilling to leave
their nests during very hot weather,
when the sun was shining brightly,
as if their eggs would be thus
injured, than during cool, cloudy,
or rainy weather.
*(2) I may specify, as instances
of dull-coloured birds building
concealed nests, the species
belonging to eight Australian
genera described in Gould's Handbook
of the Birds of Australia, vol.
i., pp. 340, 362, 365, 383, 387,
389, 391, 414.
*(3) Mr. C. Horne, Proc. Zoolog.
Soc., 1869. p. 243.
*(4) On the nidification and
colours of these latter species,
see Gould's Handbook of the Birds
of Australia, vol. i., pp. 504,
527.
If we look to the birds of England
we shall see that there is no
close and general relation between
the colours of the female and
the nature of the nest which
is constructed. About forty of
our British birds (excluding
those of large size which could
defend themselves) build in holes
in banks, rocks, or trees, or
construct domed nests. If we
take the colours of the female
goldfinch, bullfinch, or black-bird,
as a standard of the degree of
conspicuousness, which is not
highly dangerous to the sitting
female, then out of the above
forty birds the females of only
twelve can be considered as conspicuous
to a dangerous degree, the remaining
twenty-eight being inconspicuous.*
Nor is there any close relation
within the same genus between
a well-pronounced difference
in colour between the sexes,
and the nature of the nest constructed.
Thus the male house sparrow (Passer
domesticus) differs much from
the female, the male tree-sparrow
(P. montanus) hardly at all,
and yet both build well-concealed
nests. The two sexes of the common
fly-catcher (Muscicapa grisola)
can hardly be distinguished,
whilst the sexes of the pied
fly-catcher (M. luctuosa) differ
considerably, and both species
build in holes or conceal their
nests. The female blackbird (Turdus
merula) differs much, the female
ring-ouzel (T. torquatus) differs
less, and the female common thrush
(T. musicus) hardly at all from
their respective males; yet all
build open nests. On the other
hand, the not very distantly-allied
water-ouzel (Cinclus aquaticus)
builds a domed nest, and the
sexes differ about as much as
in the ring-ouzel. The black
and red grouse (Tetrao tetrix
and T. scoticus) build open nests
in equally well-concealed spots,
but in the one species the sexes
differ greatly, and in the other
very little.
* I have consulted, on this
subject, Macgillivray's British
Birds, and though doubts may
be entertained in some cases
in regard to the degree of concealment
of the nest, and to the degree
of conspicuousness of the female,
yet the following birds, which
all lay their eggs in holes or
in domed nests, can hardly be
considered, by the above standard,
as conspicuous: Passer, 2 species;
Sturnus, of which the female
is considerably less brilliant
than the male; Cinclus; Motallica
boarula (?); Erithacus (?); Fruticola,
2 sp.; Saxicola; Ruticilla, 2
sp.; Sylvia, 3 sp.; Parus, 3
sp.; Mecistura anorthura; Certhia;
Sitta; Yunx; Muscicapa, 2 sp.;
Hirundo, 3 sp.; and Cypselus.
The females of the following
12 birds may be considered as
conspicuous according to the
same standard, viz., Pastor,
Motacilla alba, Parus major and
P. caeruleus, Upupa, Picus, 4
sp., Coracias, Alcedo, and Merops.
Notwithstanding the foregoing
objections, I cannot doubt, after
reading Mr. Wallace's excellent
essay, that looking to the birds
of the world, a large majority
of the species in which the females
are conspicuously coloured (and
in this case the males with rare
exceptions are equally conspicuous),
build concealed nests for the
sake of protection. Mr. Wallace
enumerates* a long series of
groups in which this rule bolds
good; but it will suffice here
to give, as instances, the more
familiar groups of kingfishers,
toucans, trogons, puff-birds
(Capitonidae), plantain-eaters
(Musophagae, woodpeckers, and
parrots. Mr. Wallace believes
that in these groups, as the
males gradually acquired through
sexual selection their brilliant
colours, these were transferred
to the females and were not eliminated
by natural selection, owing to
the protection which they already
enjoyed from their manner of
nidification. According to this
view, their present manner of
nesting was acquired before their
present colours. But it seems
to me much more probable that
in most cases, as the females
were gradually rendered more
and more brilliant from partaking
of the colours of the male, they
were gradually led to change
their instincts (supposing that
they originally built open nests),
and to seek protection by building
domed or concealed nests. No
one who studies, for instance,
Audubon's account of the differences
in the nests of the same species
in the northern and southern
United States,*(2) will feel
any great difficulty in admitting
that birds, either by a change
(in the strict sense of the word)
of their habits, or through the
natural selection of so-called
spontaneous variations of instinct,
might readily be led to modify
their manner of nesting.
* Journal of Travel, edited
by A. Murray, vol. i., p. 78.
*(2) See many statements in
the Ornithological Biography.
See also some curious observations
on the nests of Italian birds
by Eugenio Bettoni, in the Atti
della Societa Italiana, vol.
xi., 1869, p. 487.
This way of viewing the relation,
as far as it holds good, between
the bright colours of female
birds and their manner of nesting,
receives some support from certain
cases occurring in the Sahara
Desert. Here, as in most other
deserts, various birds, and many
other animals, have had their
colours adapted in a wonderful
manner to the tints of the surrounding
surface. Nevertheless there are,
as I am informed by the Rev.
Mr. Tristram, some curious exceptions
to the rule; thus the male of
the Monticola cyanea is conspicuous
from his bright blue colour,
and the female almost equally
conspicuous from her mottled
brown and white plumage; both
sexes of two species of Dromolaea
are of a lustrous black; so that
these three species are far from
receiving protection from their
colours, yet they are able to
survive, for they have acquired
the habit of taking refuge from
danger in holes or crevices in
the rocks.
With respect to the above groups
in which the females are conspicuously
coloured and build concealed
nests, it is not necessary to
suppose that each separate species
had its nidifying instinct specially
modified; but only that the early
progenitors of each group were
gradually led to build domed
or concealed nests, and afterwards
transmitted this instinct, together
with their bright colours, to
their modified descendants. As
far as it can be trusted, the
conclusion is interesting, that
sexual selection together with
equal or nearly equal inheritance
by both sexes, have indirectly
determined the manner of nidification
of whole groups of birds.
According to Mr. Wallace, even
in the groups in which the females,
from being protected in domed
nests during incubation, have
not had their bright colours
eliminated through natural selection,
the males often differ in a slight,
and occasionally in a considerable
degree from the females. This
is a significant fact, for such
differences in colour must be
accounted for by some of the
variations in the males having
been from the first limited in
transmission to the same sex;
as it can hardly be maintained
that these differences, especially
when very slight, serve as a
protection to the female. Thus
all the species in the splendid
group of the trogons build in
holes; and Mr. Gould gives figures*
of both sexes of twenty-five
species, in all of which, with
one partial exception, the sexes
differ sometimes slightly, sometimes
conspicuously, in colour,- the
males being always finer than
the females, though the latter
are likewise beautiful. All the
species of kingfishers build
in holes, and with most of the
species the sexes are equally
brilliant, and thus far Mr. Wallace's
rule holds good; but in some
of the Australian species the
colours of the females are rather
less vivid than those of the
male; and in one splendidly-coloured
species, the sexes differ so
much that they were at first
thought to be specifically distinct.*(2)
Mr. R. B. Sharpe, who has especially
studied this group, has shewn
me some American species (Ceryle)
in which the breast of the male
is belted with black. Again,
in Carcineutes, the difference
between the sexes is conspicuous:
in the male the upper surface
is dull-blue banded with black,
the lower surface being partly
fawn-coloured, and there is much
red about the head; in the female
the upper surface is reddish-brown
banded with black, and the lower
surface white with black markings
It is an interesting fact, as
shewing how the same peculiar
style of sexual colouring often
characterises allied forms, that
in three species of Dacelo the
male differs from the female
only in the tail being dull-blue
banded with black, whilst that
of the female is brown with blackish
bars; so that here the tail differs
in colour in the two sexes in
exactly the same manner as the
whole upper surface in the two
sexes of Carcineutes.
* See his Monograph of the Trogonidae,
1st edition.
*(2) Namely, Cyanalcyon. Gould's
Handbook of the Birds of Australia,
vol. i., p. 133; see, also, pp.
130, 136.
With parrots,
which likewise build in holes,
we find analogous
cases: in most of the species,
both sexes are brilliantly coloured
and indistinguishable, but in
not a few species the males are
coloured rather more vividly
than the females, or even very
differently from them. Thus,
besides other strongly-marked
differences, the whole under
surface of the male king lory
(Aprosmictus scapulatus) is scarlet,
whilst the throat and chest of
the female is green tinged with
red: in the Euphema splendida
there is a similar difference,
the face and wing coverts moreover
of the female being of a paler
blue than in the male.* In the
family of the tits (Parinae),
which build concealed nests,
the female of our common blue
tomtit (Parus caeruleus), is "much
less brightly coloured" than
the male: and in the magnificent
sultan yellow tit of India the
difference is greater.*(2)
* Every gradation of difference
between the sexes may be followed
in the parrots of Australia.
See Gould, op. cit., vol. ii.,
pp. 14-102.
*(2) Macgillivray's British
Birds, vol. ii., p. 433. Jerdon,
Birds of India, vol. ii., p.
282.
Again, in the great group of
the woodpeckers,* the sexes are
generally nearly alike, but in
the Megapicus validus all those
parts of the head, neck, and
breast, which are crimson in
the male are pale brown in the
female. As in several woodpeckers
the head of the male is bright
crimson, whilst that of the female
is plain, it occurred to me that
this colour might possibly make
the female dangerously conspicuous,
whenever she put her head out
of the hole containing her nest,
and consequently that this colour,
in accordance with Mr. Wallace's
belief, had been eliminated.
This view is strengthened by
what Malherbe states with respect
to Indopicus carlotta; namely,
that the young females, like
the young males, have some crimson
about their heads, but that this
colour disappears in the adult
female, whilst it is intensified
in the adult male. Nevertheless
the following considerations
render this view extremely doubtful:
the male takes a fair share in
incubation,*(2) and would be
thus almost equally exposed to
danger; both sexes of many species
have their heads of an equally
bright crimson; in other species
the difference between the sexes
in the amount of scarlet is so
slight that it can hardly make
any appreciable difference in
the danger incurred; and lastly,
the colouring of the head in
the two sexes often differs slightly
in other ways.
* All the following facts are
taken from M. Malherbe's magnificent
Monographie des Picidees, 1861.
*(2) Audubon's Ornithological
Biography, vol. ii., p. 75; see
also the Ibis, vol. i., p. 268.
The cases, as yet given, of
slight and graduated differences
in colour between the males and
females in the groups, in which
as a general rule the sexes resemble
each other, all relate to species
which build domed or concealed
nests. But similar gradations
may likewise be observed in groups
in which the sexes as a general
rule resemble each other, but
which build open nests.
As I have before instanced the
Australian parrots, so I may
here instance, without giving
any details, the Australian pigeons.*
It deserves especial notice that
in all these cases the slight
differences in plumage between
the sexes are of the same general
nature as the occasionally greater
differences. A good illustration
of this fact has already been
afforded by those kingfishers
in which either the tail alone
or the whole upper surface of
the plumage differs in the same
manner in the two sexes. Similar
cases may be observed with parrots
and pigeons. The differences
in colour between the sexes of
the same species are, also, of
the same general nature as the
differences in colour between
the distinct species of the same
group. For when in a group in
which the sexes are usually alike,
the male differs considerably
from the female, he is not coloured
in a quite new style. Hence we
may infer that within the same
group the special colours of
both sexes when they are alike,
and the colours of the male,
when he differs slightly or even
considerably from the female,
have been in most cases determined
by the same general cause; this
being sexual selection.
* Gould's Handbook of the Birds
of Australia, vol. ii., pp. 109-149.
It is not probable, as has already
been remarked, that differences
in colour between the sexes,
when very slight, can be of service
to the female as a protection.
Assuming, however, that they
are of service, they might be
thought to be cases of transition;
but we have no reason to believe
that many species at any one
time are undergoing change. Therefore
we can hardly admit that the
numerous females which differ
very slightly in colour from
their males are now all commencing
to become obscure for the sake
of protection. Even if we consider
somewhat more marked sexual differences,
is it probable, for instance,
that the head of the female chaffinch,-
the crimson on the breast of
the female bullfinch,- the green
of the female greenfinch,- the
crest of the female golden-crested
wren, have all been rendered
less bright by the slow process
of selection for the sake of
protection? I cannot think so;
and still less with the slight
differences between the sexes
of those birds which build concealed
nests. On the other hand, the
differences in colour between
the sexes, whether great or small,
may to a large extent be explained
on the principle of the successive
variations, acquired by the males
through sexual selection, having
been from the first more or less
limited in their transmission
to the females. That the degree
of limitation should differ in
different species of the same
group will not surprise any one
who has studied the laws of inheritance,
for they are so complex that
they appear to us in our ignorance
to be capricious in their action.*
* See remarks to this effect
in Variation of Animals and Plants
under Domestication, vol. ii.,
chap. xii.
As far as I can discover there
are few large groups of birds
in which all the species have
both sexes alike and brilliantly
coloured, but I hear from Mr.
Sclater, that this appears to
be the case with the Musophagae
or plantain-eaters. Nor do I
believe that any large group
exists in which the sexes of
all the species are widely dissimilar
in colour: Mr. Wallace informs
me that the chatterers of S.
America (Cotingidae) offer one
of the best instances; but with
some of the species, in which
the male has a splendid red breast,
the female exhibits some red
on her breast; and the females
of other species shew traces
of the green and other colours
of the males. Nevertheless we
have a near approach to close
sexual similarity or dissimilarity
throughout several groups: and
this, from what has just been
said of the fluctuating nature
of inheritance, is a somewhat
surprising circumstance. But
that the same laws should largely
prevail with allied animals is
not surprising. The domestic
fowl has produced a great number
of breeds and sub-breeds, and
in these the sexes generally
differ in plumage; so that it
has been noticed as an unusual
circumstance when in certain
sub-breeds they resemble each
other. On the other hand, the
domestic pigeon has likewise
produced a vast number of distinct
breeds and sub-breeds, and in
these, with rare exceptions,
the two sexes are identically
alike.
Therefore if other species of
Gallus and Columba were domesticated
and varied, it would not be rash
to predict that similar rules
of sexual similarity and dissimilarity,
depending on the form of transmission,
would hold good in both cases.
In like manner the same form
of transmission has generally
prevailed under nature throughout
the same groups, although marked
exceptions to this rule occur.
Thus within the same family or
even genus, the sexes may be
identically alike, or very different
in colour. Instances have already
been given in the same genus,
as with sparrows, flycatchers,
thrushes and grouse. In the family
of pheasants the sexes of almost
all the species are wonderfully
dissimilar, but are quite alike
in the eared pheasant or Crossoptilon
auritum. In two species of Chloephaga,
a genus of geese, the male cannot
be distinguished from the females,
except by size; whilst in two
others, the sexes are so unlike
that they might easily be mistaken
for distinct species.*
* The Ibis, vol. vi., 1864,
p. 122.
The laws of
inheritance can alone account
for the following
cases, in which the female acquires,
late in life, certain characters
proper to the male, and ultimately
comes to resemble him more or
less completely. Here protection
can hardly have come into play.
Mr. Blyth informs me that the
females of Oriolus melanocephalus
and of some allied species, when
sufficiently mature to breed,
differ considerably in plumage
from the adult males; but after
the second or third moults they
differ only in their beaks having
a slight greenish tinge. In the
dwarf bitterns (Ardetta), according
to the same authority, "the male
acquires his final livery at
the first moult, the female not
before the third or fourth moult;
in the meanwhile she presents
an intermediate garb, which is
ultimately exchanged for the
same livery as that of the male." So
again the female Falco peregrinus
acquires her blue plumage more
slowly than the male. Mr. Swinhoe
states that with one of the drongo
shrikes (Dicrurus macrocercus)
the male, whilst almost a nestling,
moults his soft brown plumage
and becomes of a uniform glossy
greenish-black; but the female
retains for a long time the white
striae and spots on the axillary
feathers; and does not completely
assume the uniform black colour
of the male for three years.
The same excellent observer remarks
that in the spring of the second
year the female spoon-bill (Platalea)
of China resembles the male of
the first year, and that apparently
it is not until the third spring
that she acquires the same adult
plumage as that possessed by
the male at a much earlier age.
The female Bombycilla carolinensis
differs very little from the
male, but the appendages, which
like beads of red sealing-wax
ornament the wing-feathers,*
are not developed in her so early
in life as in the male. In the
male of an Indian parrakeet (Paloeornis
javanicus) the upper mandible
is coral-red from his earliest
youth, but in the female, as
Mr. Blyth has observed with caged
and wild birds, it is at first
black and does not become red
until the bird is at least a
year old, at which age the sexes
resemble each other in all respects.
Both sexes of the wild turkey
are ultimately furnished with
a tuft of bristles on the breast,
but in two-year-old birds the
tuft is about four inches long
in the male and hardly apparent
in the female; when, however,
the latter has reached her fourth
year, it is from four to five
inches in length.*(2)
* When the male
courts the female, these ornaments
are vibrated,
and "are shewn off to great advantage," on
the outstretched wings: A. Leith
Adams, Field and Forest Rambles,
1873, p. 153.
*(2) On Ardetta, Translation
of Cuvier's Regne Animal, by
Mr. Blyth, footnote, p. 159.
On the peregrine falcon, Mr.
Blyth, in Charlesworth's Mag.
of Nat. Hist., vol. i., 1837,
p. 304. On Dicrurus, Ibis, 1863,
p. 44. On the Platalea, Ibis,
vol. vi., 1864, p. 366. On the
Bombycilla, Audubon's Ornitholog.
Biography, vol. i., p. 229. On
the Palaeornis, see, also, Jerdon,
Birds of India, vol. i., p. 263.
On the wild turkey, Audubon,
ibid., vol. i., p. 15; but I
hear from Judge Caton that in
Illinois the female very rarely
acquires a tuft. Analogous cases
with the females of Petrcocssyphus
are given by Mr. R. Sharpe, Proeedings
of the Zoological Society, 1872,
p. 496.
These cases must not be confounded
with those where diseased or
old females abnormally assume
masculine characters, nor with
those where fertile females,
whilst young, acquire the characters
of the male, through variation
or some unknown cause.* But all
these cases have so much in common
that they depend, according to
the hypothesis of pangenesis,
on gemmules derived from each
part of the male being present,
though latent, in the female;
their development following on
some slight change in the elective
affinities of her constituent
tissues.
* Of these latter cases Mr.
Blyth has recorded (Translation
of Cuvier's Regne Animal, p.
158) various instances with Lanius,
Ruticilla, Linaria, and Anas.
Audubon has also recorded a similar
case (Ornitholog. Biography,
vol. v., p. 519) with Pyranga
aestiva.
A few words
must be added on changes of
plumage in relation
to the season of the year. From
reasons formerly assigned there
can be little doubt that the
elegant plumes, long pendant
feathers, crests, &c., of egrets,
herons, and many other birds,
which are developed and retained
only during the summer, serve
for ornamental and nuptial purposes,
though common to both sexes.
The female is thus rendered more
conspicuous during the period
of incubation than during the
winter; but such birds as herons
and egrets would be able to defend
themselves. As, however, plumes
would probably be inconvenient
and certainly of no use during
the winter, it is possible that
the habit of moulting twice in
the year may have been gradually
acquired through natural selection
for the sake of casting off inconvenient
ornaments during the winter.
But this view cannot be extended
to the many waders, whose summer
and winter plumages differ very
little in colour. With defenceless
species, in which both sexes,
or the males alone, become extremely
conspicuous during the breeding-season,-
or when the males acquire at
this season such long wing or
tail-feathers as to impede their
flight, as with Cosmetornis and
Vidua,- it certainly at first
appears highly probable that
the second moult has been gained
for the special purpose of throwing
off these ornaments. We must,
however, remember that many birds,
such as some of the birds of
paradise, the Argus pheasant
and peacock, do not cast their
plumes during the winter; and
it can hardly be maintained that
the constitution of these birds,
at least of the Gallinaceae,
renders a double moult impossible,
for the ptarmigan moults thrice
in the year.* Hence it must be
considered as doubtful whether
the many species which moult
their ornamental plumes or lose
their bright colours during the
winter, have acquired this habit
on account of the inconvenience
or danger which they would otherwise
have suffered.
* See Gould's Birds of Great
Britain.
I conclude, therefore, that
the habit of moulting twice in
the year was in most or all cases
first acquired for some distinct
purpose, perhaps for gaining
a warmer winter covering; and
that variations in the plumage
occurring during the summer were
accumulated through sexual selection,
and transmitted to the offspring
at the same season of the year;
that such variations were inherited
either by both sexes or by the
males alone, according to the
form of inheritance which prevailed.
This appears more probable than
that the species in all cases
originally tended to retain their
ornamental plumage during the
winter, but were saved from this
through natural selection, resulting
from the inconvenience or danger
thus caused.
I have endeavoured in this chapter
to shew that the arguments are
not trustworthy in favour of
the view that weapons, bright
colours, and various ornaments,
are now confined to the males
owing to the conversion, by natural
selection, of the equal transmission
of characters to both sexes,
into transmission to the male
sex alone. It is also doubtful
whether the colours of many female
birds are due to the preservation,
for the sake of protection, of
variations which were from the
first limited in their transmission
to the female sex. But it will
be convenient to defer any further
discussion on this subject until
I treat, in the following chapter,
of the differences in plumage
between the young and old. |