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Descent of Man [ 1871 ]

Charles Darwin [ 1809 - 1882 ]

 

Chapter XV - Birds- Continued


  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.

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