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Chapter XV
Birds--continued.
Discussion as to why the males alone of some species, and both sexes of
others, are brightly coloured--On sexually-limited inheritance, as applied
to various structures and to brightly-coloured plumage--Nidification in
relation to colour--Loss of nuptial plumage during the winter.
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' (1. Fourth edition, 1866, p. 241.) 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 coloration has been
discussed in some very interesting papers by Mr. Wallace (2. 'Westminster
Review,' July 1867. 'Journal of Travel,' vol. i. 1868, p. 73.), 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.
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, etc., 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. (3. 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 (4. Dr. Chapuis, 'Le Pigeon
Voyageur Belge,' 1865, p. 87.) in which the males alone are marked with
black striae. So again Mr. Tegetmeier has recently shewn (5. The 'Field,'
Sept. 1872.) 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.
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 (6. Bechstein, 'Naturgeschichte Deutschlands,' 1793, B.
iii. 339.) 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.
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. (7. 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.) 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.
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 (8.
Mr. Ramsay, in 'Proc. Zoolog. Soc.' 1868, p. 50.) 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.
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
coloration 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 (9. 'Journal of Travel,' edited by A. Murray, vol. i. 1868, p.
78.), 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.
There is in the first place much truth in the Duke of Argyll's remark (10.
'Journal of Travel,' edited by A. Murray, vol. i. 1868, p. 281.) 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 (11. Audubon, 'Ornithological Biography,' vol. i. p.
233.), 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.
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 (12.
Jerdon, 'Birds of India,' vol. ii. p. 108. Gould's 'Handbook of the Birds
of Australia,' vol. i. p. 463.); 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. (13. 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 (14. 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.); so that it is no
valid objection to his view that many birds having both sexes obscurely
coloured build concealed nests. (15. 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.) 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 (16. Mr. C. Horne,
'Proc. Zoolog. Soc.' 1869. p. 243.); 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. (17. On the
nidification and colours of these latter species, see Gould's 'Handbook to
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 blackbird, 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. (18. 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.) 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.
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
(19. 'Journal of Travel,' edited by A. Murray, vol. i. p. 78.) a long
series of groups in which this rule holds 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 (20. 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.), 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.
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 (21. See his Monograph of the
Trogonidae, 1st edition.) 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.
(22. Namely, Cyanalcyon, Gould's 'Handbook to the Birds of Australia,'
vol. i. p. 133; see, also, pp. 130, 136.) 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.
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. (23. Every gradation of
difference between the sexes may be followed in the parrots of Australia.
See Gould's 'Handbook,' etc., vol. ii. pp. 14-102.) 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. (24. Macgillivray's 'British Birds,' vol. ii. p. 433. Jerdon,
'Birds of India,' vol. ii. p. 282.)
Again, in the great group of the woodpeckers (25. All the following facts
are taken from M. Malherbe's magnificent 'Monographie des Picidees,'
1861.), 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 (26. Audubon's
'Ornithological Biography,' vol. ii. p. 75; see also the 'Ibis,' vol. i. p.
268.), 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.
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. (27. Gould's
'Handbook to the Birds of Australia,' vol. ii. pp. 109-149.) 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.
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. (28. 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, fly-
catchers, 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. (29. 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
(30. 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.), are not developed in her
so early in life as in the male. In the male of an Indian parrakeet
(Palaeornis 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. (31. 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 Petrocossyphus are given by Mr. R.
Sharpe, 'Proceedings 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. (32. 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.) 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.
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, etc., 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. (33. See Gould's 'Birds of Great Britain.') 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.
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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